Given their history? Probably a multi hit boss mechanic with no indicator that they’re doing it (or one that’s hard to see in the chaos of spell effects) that requires each tank to only absorb 1 hit.
Everquest had several tank/melee mechanics to be aware of.
- NPCs could single hit, dual hit, or quad hit naturally (depended on NPC type) - varying the incoming rate of damage.
- NPCs could FLURRY in which they would double the amount of attacks, all at once, with no indicator. This could and did kill tanks who were not paying attention or who had inattentive healers.
- NPCs would RAMPAGE, in which they would hit the two players highest on the aggro list with full rounds of melee (think Patchwerk)
- NPCs could AE Rampage, which would they would hit every player in melee for a full combat round
- NPCs could Infuriate - in which they would riposte melee damage, and inattentive DPS would basically kill themselves from the reflected damage.
NPCs would also often have very large magic AE attacks which could seriously injure players.
Most of these had no advance notice, and you had to plan your cooldowns, tank swaps and healing around the unpredictable damage. Conversely, this increased the value of debuffs which would make these melee rounds less lethal.