Epic Spell: Allied Martyr

School: Divination [Foresight]
Components: V, S
Range: Touch
Target: One allied creature
Duration: 20 hours, or until death due to martyrdom
Saving Throw: No
Spell Resistance: No

When you cast this on an ally, you are essentially assigning that creature to be your source of health and vitality. With the ally's permission*, any subsequent damage received by you will be instantaneously transferred to the ally, for the duration.
*NPC allies automatically accept; only other players must accept the consequences freely.