Spell:Blood Bond

Birthright spell

The caster links the life force of two people, any damage suffered by one target is shared by the other. Source: BoP page 84.

The spell does not affect bloodline despite the name, instead it binds the fates of two individuals. The spell transfers half of any damage suffered by one of the people to the other, although non-damage effects (broken limbs, burns, fatigue, curses, charms, etc) do not transfer.
The spell can be cast on unwilling targets (a favorite of goblin and Vos priests), but is usually used on willing recipients - for example a Rjurik wife may be blood bound to her beloved to aid him in battle, a pair of Cuiraécenite knights may bind each other, etc.
If the spell reduces either party to less than 1 hit point then they get a second saving throw.
If the spell reduces the recipient of any transferred damage to under -10 hit points then it ceases immediately, with any 'surplus' damage that would 'normally' have been transferred instead staying with the injured party.
The blood bond does not work across planar boundaries, but otherwise distance does not affect the bond.

The spell was famously used by a junior priestess of Nesirie named Tchervy. Captured by a Vos Tsarevo as he led troops to raid Brecht settlements, after her pleas failed to deter the Tsarevo from raiding the settlements and enslaving or killing the inhabitants, Tchervy cast the spell as the Tsarevo slept and then jumped off a cliff killing them both.

Priests of Belinik are notorious for linking captives or peasants to their warriors, some Rjurik clans traditionally link a husband and wife.

As a 2e spell this is a spell of the 'combat' sphere. The 2e duration was 5 rounds/level, but that means the victim must be quite close to the battle reducing the usefulness of the spell.