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As mentioned at the outset Jaraxxus likes to summon two different kinds of adds. The first he will call is a single Mistress of Pain, who will come through a demon portal. She's a pretty wicked add. She needs to be tanked, and all DPS needs to switch and kill her. Her main attack is something of a whirlwind, but not too incredibly dangerous. She also likes to use Spinning Pain Spike on a random target. When she does this, she'll grab them, leap up into the air, and slam them down, inflicting massive amounts of damage and messing up
raid positioning. It isn't uncommon for a Pain Spike victim to get Fel Lightening chained onto them immediately after and die.
The second add type is Infernals. Jaraxxus will scream "Inferno!" and an Infernal Volcano will spawn in the room. The volcano itself deals AoE damage to anyone standing near it, but it also spews forth three Infernals which tend to just pick a person and follow them until taunted off. These infernals do basic melee attacks, but really like to stand still and do an AoE, or randomly charge someone leaving behind a trail of flame. Infernals don't have very high health, and move around too much for melee DPS to be very effective on them, so only ranged has to switch from DPSing the boss to killing the infernals.
It is worth pointing out that on Heroic modes the Mistress' Portal and the Volcanos will not stop spawning their respective adds until they're destroyed. Also, Mistresses gain a nasty spell lock ability that can totally shut down a healer and wipe the raid.