Normally Drain Soul as the tooltip states will only return mana if the monster you are using it on dies while you are using it.
I'm currently leveling in a spot where I can dot 5-6 mobs up and howl of terror them, and then I Imp drain to return some of my mana... I just found out that if you dot up all the mobs, and then cast Imp Drain soul on any one of the mobs, as the rest of the mobs die, you get mana returned for each one, even though you are only using the spell on one of the mobs.
Neat lil bug right now that could really help with your mana downtime if you're not using it effectivly.
I'm currently leveling in a spot where I can dot 5-6 mobs up and howl of terror them, and then I Imp drain to return some of my mana... I just found out that if you dot up all the mobs, and then cast Imp Drain soul on any one of the mobs, as the rest of the mobs die, you get mana returned for each one, even though you are only using the spell on one of the mobs.
Neat lil bug right now that could really help with your mana downtime if you're not using it effectivly.