Phase One abilities
Flame Breath
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.
Knock Away
Damage Onyxia's main target, knock it back a distance, and reduce her hate of the target by one quarter (25%). If this reduces hate enough, she will switch targets.
Wing Buffet
Damages and knocks back multiple targets in a cone in front of Onyxia. Unlike the Drakes in Blackwing Lair, Onyxia does not reduce her hate of targets she hits with this attack.
Cleave
Nasty physical damage to multiple targets immediately in front of Onyxia.
Tail Sweep
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
Flight
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 in November 2006 strongly 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.
Whelp Spawns
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.
There were many theories about deep breaths being caused by clumping, not enough DPS, or the number of debuffs on her, however Jeff Kaplan stated in a video interview with the developers in June 2007 that the trigger for the Deep Breath is "totally random" [1]. The Brady Games World of Warcraft Dungeon Companion confirms that she has a random chance to perform a Deep Breath attack every time she moves.
It is human nature to find patterns in randomness, so many players insist that they know exactly what causes the deep breath attack. However, each of these theories has been tested and found to be false. There are only two techniques that have been proven to work against Deep Breath attacks: Damage her more quickly so that she has fewer opportunities to perform the attack, and move out of the way when she does perform it. See below to see how to dodge this attack.
Note that well-geared guilds can bring her through Phase Two so quickly that she only moves a couple times, and thus has a good chance of not performing the attack at all. The members of these guilds report that their techniques work because Onyxia hardly ever uses her Deep Breath attacks any more. In fact, the time spent perfecting their techniques was also time spent gathering powerful gear, so Onyxia is simply not in the air long enough to perform the attack. Anyone wishing to promote a theory about what causes the attack should test it by leaving Onyxia up in the air for at least 15 minutes, which will be long enough to collect useful information.
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).
Whelp Spawns
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.
