Mana Burn works as such say I have 50 hp, and 30 mp.
Now i have several spells; a fire ball that costs 5 mana burn, and a healing spell that costs 10 mp and heals 2d6.
Now say I cast the fire ball. I’m now at 50hp, and 25 mp.
Now I take 20 damage. I’m now at 30 hp, and 25 mp.
Now I use the healing spell and roll 2 6’s that’s +12 hp. I’m now at 50hp, and 15 mp.
Now say I’m at 50 hp, and 0 mp. Normally you can’t cast a spell but with mana burn system you can cast the spell. OK the fire ball costs 5 mp but if you have none you can the fire ball for 10 hp. But since healing spells cost mp, not mana burn you can’t abuse it.
Now say your fighting a warrior with 30 hp and 0 mp. but your using a bastard sword that is enchanted with mana burn. now say you hit and roll a 5 on your 1d10 damage roll but a 10 on your say 1d12 mana burn damage you do 25 damage to said warrior. or if he had mp say 15 he would take 10 mp damage and only 5 normal damage. now say he had 5 mp then he would take 5 mp damage and 15 hp damage.
The basic idea behind all this is mana is derived from the Æther in the world. Æther being in your body and around you mage’s using the mana there bodies refine from Æther. and the hp damage being damage in the form of using unrefined mana IE Æther that holds the body together.
If you have any further questions please feel free to ask me.