Yep, these Eredar casters that can transform into bigger demons at death if not properly interrupted. They have a cast called Mark of the Legion or something that gives the target 30% haste and 30% increased damage for 20 seconds and is on a 30 second CD, so basically an improved bloodlust on whoever you decide worthy for 60% of the time.
Cool things that happen are:
1) You're suddenly everyone's best friend because everyone wants that buff for the bosses
2) If it lines up with Gul'dan trinket, Concordance and good shard refunds, you can shoot to 2.5M DPS in the first 20 seconds of the boss fight.
Things to watch out for:
a) Enslave only lasts 5 minutes. Especially on higher m+, the chaos bolts of these mobs hurt badly, so watch that timer and if necessary release and re-enslave in advance of it running out
b) When you die in a fight, the enslave breaks and you'll have another mob to fight. So don't die
c) The pathing of these mobs is atrocious. You won't be able to skip any mob packs, even if they are cc'ed or cut corners by jumping because that stupid enslave has a rather large aggro radius
d) Lastly and most annoyingly: At the final boss, the enslave will break during the intermission phase (when you are teleported and have to run back) and in 8 of 10 cases, the boss just resets after a while during this phase. So make sure to get the buff one last time and release & kill the mob before pulling last boss.