Blood Magic
For the support gem, see Blood Magic (Support Gem).
The Blood Magic keystone passive skill gives the following effects:
Skills that reserve mana will now reserve life.
Eldritch Battery will not give you back your mana, it will merely remove your Energy Shield.
When combined with Chaos Inoculation, casting a spell will kill the player instantly.
Casting a spell that would reserve 100% of your mana with this keystone will also kill the player instantly. For example Spell Totem + Determination (250% cost increase of a 40% reserve aura for a 100% reserve cost, which will reserve all your health)