When Lord Jaraxxus spawns his Mistresses, they will cast Mistress' Kiss upon two people (on 10-man) at once (it will reach one person, and then a second or two later another person will get the debuff as well). This is a DEBUFF that will last for 15 seconds and is really harmless... until you cast something with a cast time ;) At that point, it'll interrupt your spell and lock you out of all spells of that school of magic for 8 seconds!
You can add this debuff to your Grid frames, and watch for it. Target the demon when she spawns so you'll know when she casts the debuff, or have your OT call it out; it cannot be interrupted, but it's good to know when it's going off. When it goes off, the raid needs to be alerted to hold off on spell casts (timed ones) until both of the debuffs have been assigned. Once the two kisses have been applied--it can and will hit ret, prot, and feral pallies and druids--the untouched players of the raid can continue casting as normal.
IF YOU HAVE THE KISS:
The best options are to either:
- Use a timed spell of a school you don't normally use (such as starfire for a resto druid) to trigger it, or
- Wait it out for the 15 seconds while using only insta-cast spells.
- This pretty much screws over holy pallies; holy pallies might as well trigger the debuff and wait the 8 seconds for the spell lock to be over rather than wait the 15 seconds for the debuff to wear off!
We used 3 healers for killing heroic Jaraxxus on 10-man (holy priest, resto shaman, resto druid), for those curious. We used 2 for the Beasts, because we needed the extra dps.