The Servant will cast HOTs, mana regen, and haste.
If you have a healer merc, the merc will NOT cast heals if there is a HOT from the Servant currently on. We found this out the hard way when the Servant couldn't keep up with the beating a PC was taking, and the merc just stood there until the PC was dead. Then, in an apparent fit of remorse, the merc tried to raise the PC.
Tip: Just before the end of your half hour, kill the mana regen and haste. The Servant will recast them, and the effects will continue for their normal time, about 15 minutes.