Blood Binding

You attempt to bind the blood of two creatures, so that they take equivalent injuries. Both targets must each make a Constitution saving throw. If the blood of a target is used in the casting of Blood Binding, that target has Disadvantage on the Constitution saving throw to resist the effects of the spell. Both creatures must fail the saving throw in order for the spell to succeed. If one target fails and the other succeeds, you may attempt to bind a replacement creature within range, otherwise the spell fails. 

If the spell succeeds, what injures one target injures the other; whenever one creature takes HP damage, the other creature takes the same amount of HP damage. The spell lasts until the end of the round or until one of the targets dies.

The spell works on creatures that have blood or a blood equivalent, including plants with sap, some constructs like flesh golems, and undead like vampires. Creatures with no blood or blood equivalent, such as oozes or animated skeletons, are unaffected by this spell.

At higher levels. Casting this spell at higher levels lengthens the effect of the spell; 1 minute at 4th level, 1 hour at 5th level, 1 day at 6th level, 1 week at 7th level, 1 month at 8th level, and 1 year at 9th level.