Maybe something like Ambitious Sacrifice, but dealing constant damage on conjurer which gives you mana ? You sacrifice your health to get more mana, but have to take care about self hp. Great inside forts, outside it with knight next to you (auras) would also work.
Lets say, it takes 50 hp every 5 seconds and gives you 100 mana every "bit". It's like suicide
Regen self would be usefull again.
Spell activated like recharged arrows, ends when you have below 25% hp.