Onyxia is immune to all abilities which cause a monster to instantly target the caster. So, abilities such as [Taunt], [Challenging Shout], and [Mocking Blow] don't work as usual on her. Other threat-generating abilities such as [Sunder Armor] do work as usual.
Onyxia is immune to all fire spells and effects.
At the East and West sides of Onyxia's Lair are the two entrances to a tunnel full of eggs. When a character gets too close to the eggs, many whelps will hatch and attack.
As soon as Onyxia is engaged, her Onyxian Warders respawn in the tunnel. They despawn as soon as she is defeated. These Warders discourage players from hiding from her in the tunnel.
As with many other raid bosses, Onyxia will teleport characters to herself if they are far away or unreachable when she is in combat. This usually happens to characters in the entrance tunnel, but can randomly happen to characters at any position in her lair.
Phase One abilities
This is a blast of fire that hits everyone in front of Onyxia, including to a considerable distance to the sides. This damage is fire based and hits for upwards of 4000 damage.
Damages her main target, knocking it back a distance, and reducing the threat of the target by one quarter (25%).
Damages and knocks back multiple targets in a cone in front of Onyxia.
Nasty physical damage to multiple targets immediately in front of Onyxia.
Onyxia can attack everyone behind her with her mighty tail. This hits for very little damage (about 300) but sends the target flying sideways. This attack can cause a wipe if the victim gets knocked into a whelp cave.
Phase Two abilities
In the second stage of the Onyxia fight, she takes flight. Normal melee auto-attacks cannot reach her. Special melee attacks (rogues ambush, sinister strike, eviscerate...) can reach her.
Engulfing Flames (Fireball)
While in flight Onyxia will breathe fireballs down on random players. This hits as a main hit and a splash damage hit. It does up to 3000 damage to the player it hits and can do up to 1500 to anyone nearby. Note: Research indicates that these fireballs target whoever is highest on Onyxia's threat list, and impose a large (probably complete) threat wipe for the characters targeted.
Groups of whelps periodically hatch from the eggs and attack, regardless of whether any characters are near them. This includes a large spawn at the start of the phase, and small spawns periodically during the phase.
Onyxia's Breath (Deep Breath)
Onyxia emotes "Onyxia takes in a deep breath..." and a few seconds later, she flies across her chamber as she breathes a huge gout of fire that rolls straight across the room from her initial position to the opposite wall. Characters in the middle of this attack get hit five or six times and are likely to die. Characters on the sides of the attack get missed entirely or get hit only once.
Despite being "breaths" they don't actually originate from the front of her — they actually originate from the floor directly beneath her and travel directly to the opposite corner or side of her lair. Melee who attempt to stand under her to DPS should keep this in mind and be prepared to immediately run to a safe spot if a deep breath occurs.
Phase Three abilities
Phase One Abilities
During Phase Three, Onyxia uses all of her Phase One abilities again.
AOE Fear (Bellowing Roar) / Lava Crack Eruptions
Onyxia does an AoE fear at random intervals, causing everyone to run around randomly for a few seconds. Sometimes there are 30 seconds between fears and sometimes as little as 10 seconds. Just before she casts the fear, the screen shakes. After the fear is over, the lava cracks spew forth lava for a few seconds. Characters hit with the lava take about 1,500 points of fire damage (unresisted). Keeping Fear ward on the Tanks is advised.
Small groups of whelps occasionally spawn from the eggs and attack, regardless of whether any characters are near them. These spawns are rare, unlike the common spawns in Phase Two. These spawns are comprised of five or so whelps, unlike the huge spawns of 20+ whelps that result when characters are feared or knocked into the eggs. These spawns usually appear early in Phase Three, and may be delayed effects of the normal whelp generation in Phase Two.
Phase One strategies
Phase One starts as soon as anyone enters Onyxia's main chamber. There are three main goals for Phase One:
Establish and maintain aggro on the main tank.
Slow (auto-attack and non-mana) damage until Onyxia enters Phase Two.
Entering and positioning
When everyone is ready, the Main Tank charges in with everyone else following about two or three seconds behind. The main tank runs straight past Onyxia, pulling her to the far north side of the room so that she faces the wall. The best positioning takes a little practice and is a few steps in front of the wall. This is so that the main tank's back is to the wall to limit the effectiveness of her Knock Away ability and so that the main tank is the only one in front of her when she breathes fire.
Everyone else must charge in two or three seconds after the main tank. If they wait longer, there is a danger of attracting attention from a freshly-spawned Onyxian Warder. If they go sooner, they run the risk of getting hit by Onyxia's Flame Breath attack. Don't get ahead of her!
Everyone but the main tank splits to the sides to the northeast and northwest "corners" of Onyxia's chamber. Each side forms three main groups. Closest to Onyxia is the melee group, roughly at her shoulders. Next is the ranged and healing group, which should be right along the wall. Last, and farthest south, is the whelp group.
Establish Main Tank threat and initiate slow damage
Initially, nobody attacks Onyxia except the main tank. The main tank is the only one to be fighting Onyxia for at least a minute, or until she drops to 99% health (whichever comes first).
At this point, the DPS characters start doing damage while being careful to avoid drawing aggro away from the main tank. This is not a race. Phase One is a slow, steady fight. Do not try to rush. All DPS characters try for about 50-75% of their max DPS output. Anyone with a way to dump aggro does so (e.g. Hunters can [Feign Death], Rogues can [Feint]) as often as they can. Mages and warlocks use wands to save mana and generate minimal threat (having over 90% mana going into Phase Two is helpful).
Remember that Onyxia's Flame Breath can do up to 4000 damage. Remember this, because if anyone draws aggro away from the main tank, she turns sideways and may breathe. If she does, she can kill all cloth wearers (casters and healers) on that side, very likely causing a wipe. Therefore it is very important to not overdo the damage.
Also remember that Onyxia's Knock Away ability reduces her threat on the main tank. The main tank is unable to hold aggro as well as usual, so the DPS classes must hold back on their damage output.
If, despite this warning, you pull aggro off of the main tank and Onyxia attacks you, attempt to shake her off of you with [Feign Death], [Vanish], [Ice Block], [Divine Shield], [Divine Protection], or [Fade]. If that doesn't work, run to the tanking spot at the northern end of the chamber and fight her there. You are going to die anyway, and this will keep Onyxia from breathing flame on other characters as she kills you.
Phase Two strategies
Phase Two starts when Onyxia reaches 65% health. She walks towards the south end of her lair and takes flight.
There are four basic goals for Phase Two:
Reduce damage from Deep Breath and fireball attacks.
Keep whelps under control.
Damage Onyxia as quickly as possible.
Prepare for Phase Three.
In this phase you must ensure that everyone maintains spacing from each other to reduce blast damage from fireballs. Players need to move around to ensure they have range to Onyxia while she is flying. Ideally you want to be to on either side of the line created by her head-tail axis as her breath always flows through it. This will minimize the distance you need to run to avoid a deep breath should it occur. It's also wise to turn on an aggro warning mod to warn you when you're about to be fireballed. There is always a 2–5 second delay between her targeting someone and actually shooting the fireball. Once targeted, skilled players can distance themselves from teammates to help avoid collateral damage.
When Onyxia takes off, each whelp pit will spawn a large group of whelps that need to be dealt with quickly. The raid should help the whelp groups clear this initial spawning as fast as possible before moving on to help with Onyxia. Since the first spawn is quite large and can kill clothies quite easly, it is advised to put a tank right at the entrance to the whelps pit, who will cast [Challenging Shout] to aggro the whelps to him. Then the mages can kill those easily. After the initial group is dead the whelps spawn in regular intervals in smaller numbers that can be dealt with fairly easily once you are used to them.
Dodge deep breath attacks
The path of Onyxia's Deep Breath ability is dependent on her position.
During Phase Two, Onyxia has six possible positions in her cave and stays in each position long enough to fire five fireballs. After that, she randomly moves clockwise, counterclockwise or across her chamber. If she chooses to move across her chamber, she will perform a Deep Breath attack on the way. She will emote, "Onyxia takes in a deep breath..." There are a slew of AddOns which announce this across a player's screen.
Upon this announcement, everyone must immediately run to a safe position.
For each of the six possible positions she moves to there are always two positions in which the raid can stand and be safe from the deep breaths. If she's in a corner position, then the two safe positions are the adjacent corners and if she's in a side position then the two safe positions are the adjacent sides. For example if she's in the top-left corner then the top-right and bottom-left corners are both safe and if she's at the top-center then the two safe positions are the left-center and right-center. Anyone not in a safe position usually dies during a deep breath unless they get lucky with resisting the fire damage.
It is useful to have hunters use Track Dragonkin, ping her position on the mini-map, and announce each time she changes position so that everyone can then adjust their position. Ideally, everyone remains fairly close to a safe position at all times during Phase Two.
Max out damage
Get Onyxia down to about 40% health as fast as you can. Ranged damage is the main thing that can hit her while in flight, so it is important to have everyone doing as much ranged DPS as possible. Rogues and other melee can hit her by using special attacks. Track all debuffs and DoTs on her and ensure they are kept up to move through this phase as quickly as possible. Onyxia's targeting does not follow the usual rules during Phase Two, so there is no reason to hold back on damage. The more quickly Phase Two ends the fewer opportunities Onyxia will have to do a Deep Breath.
Prepare for Phase Three
Phase Two ends as Onyxia is brought to 40% health. At the start of Phase Three, she lands at a random point in her chamber and starts her ground attack. She casts a fear spell within seconds of landing, so all players must position themselves away from the whelp caves before she does so. If not, the fear effect may cause some to run into the whelp eggs, spawning a lot of whelps.
Due to Onyxia's in-flight threat wiping, the main tank will have very little threat as she lands. He must build rage on whelps, then use [Sunder Armor] and other threat-generating attacks on Onyxia shortly before she lands. Another technique is for the main tank to bandage himself as she is landing due to the large amount of aggro created by the self-healing. Casters stop attacking somewhere between 41% and 44%. Hunters and rogues keep attacking until she starts to land, at which point they Feign Death or Vanish.
Phase Three strategies
Phase Three starts as Onyxia is brought to 40% health. She lands and resumes her attack from the ground. The phase ends when she dies.
Phase Three tactics are very similar to Phase One, except that fear requires special handling and the raid will take damage from lava spurts. The main goals are as follows:
Position to reduce trouble from fear.
Establish and control main tank aggro.
Deal with fear.
Perform slow DPS.
In Phase Three everyone should move back to their Phase One positioning with some minor changes. Stand in a spot well away from the cracks that erupt lava. While this will not prevent taking damage, it will limit it somewhat. Make sure everyone is spread out and that the main tank is back to the north end of the room.
There is a considerably better spot for ranged DPS characters. At the exact center of the room is a medium-sized area with fewer lava cracks (prior to Patch 1.11, these did not spew lava at all). Ranged damage-dealers standing here can avoid all damage from Onyxia and the lava, although they may be attacked by whelps. They must stand back far enough to avoid Onyxia's tail. There are two small spikes of rock that make excellent places to stand.
Healers who are healing the main tank should stand against the wall in their corners, as far back as possible. This gives them the best chance to avoid both Onyxia's Fire Breath and Tail Swipe attacks when feared.
Melee must stay well forward of her tail! If you are too far back, she may fear you into her tail, which will toss you into a whelp pit, which will spawn 20 whelps, which will wipe the raid. Also, everyone must stay away from the whelp pits to avoid being feared directly into them. If you do end up spawning a bunch of whelps, call it out so that the whelp groups can kill them quickly. AoE damage is required to have any chance of success.
At beginning of Phase Three, the main tank may have a difficult time re-establishing aggro. Onyxia may attack and kill a half-dozen members of the raid before the main tank finally gets her attention. Getting her attention is a lot faster if, near the end of Phase Two, the main tank applies Sunder Armor and the casters stop attacking. Usually a full 1% health drop (as in Phase One) will ensure complete aggro control for the rest of the fight if people are careful with their DPS.
As in Phase One, if you draw aggro from the main tank, either shake it or run to the tanking spot to die. This is very likely to happen at the beginning of Phase Three to all cloth classes, so be ready for it.
Onyxia's threat at the start of Phase Three is a hot topic of debate. Research in November 2006 strongly indicates that her Phase Two fireball attack, when it scores a targeted (not AoE) hit, is a complete threat wipe. This would go a long way toward explaining the strange behavior at the start of Phase Three.
Dealing with fear
Onyxia's fear spell causes a considerable amount of chaos. Aside from disrupting healing and damage-dealing, it causes all characters to lose their positions and to take damage from running through lava spurts. It sometimes causes the main tank to run sideways. When this happens, Onyxia turns to follow him, and may then breathe flame on members of the raid on that side.
The cracks spew lava immediately after the fear effect ends. If your character is standing on a crack when the fear effect ends, move him to a safe spot quickly.
The fear interrupts long-cast spells, and wastes much of the duration of short-term buffs (such as from trinkets). Use these immediately after a fear effect ends, in order to reduce the chance of disruption from the next fear.
It helps to have a Priest be the main healer on the MT during Phase Three. The Priest ability [Fear Ward], when cast on the MT during Phase Three, can prevent the main tank from being feared.
The shaking screen gives just enough warning to use [Berserker Rage]. However, the reduced threat generation and extra damage taken from [Berserker Stance] while the healers are feared can be problematic.
An advanced tactic is to have two tanks, who must both build threat. The Off-Tank hits Berserker Rage to avoid the fear, and becomes the main tank until the fear is over. This requires more skill by the healers and tanks, and more fire resistance gear. However, if done properly, it ensures that Onyxia will never turn due to fear effects. The off-tank may be positioned with the main tank, to avoid repositioning, or at her shoulder, to avoid damage.
If you are not the tank, you must not avoid or break the fear effect. Onyxia will always attack un-feared targets before targets that do have the fear effect. What this means is that if you aren't feared, Onyxia may turn, breathe flame on you and many other players, and wipe the raid. Stay away from tremor totems. Don't use Will of the Forsaken or your PvP trinkets to break fear.
There is a small chance of resisting the fear. If you resist it, Onyxia will attack you. Run to the tanking spot until the main tank recovers from fear and regains aggro.
Perform slow DPS
This part of the fight is similar to Phase One. Keep your DPS under control doing about 50-75% of your max and clearing aggro as often as you can.
Damage-dealing classes can gradually ramp up their damage output as Phase Three progresses and the main tank builds more threat. Hunters can do this more quickly by using Feign Death and Rogues can do so with Vanish. At 10% health, it is usually possible to apply maximum damage. This is a balancing act, as Onyxia must be brought down before attrition from the lava spurts causes the loss of too many players, yet the damage-dealers must not pull aggro from the main tank.