Bleed!

When a creature you can see takes damage you can escalate their wounds immediately. As a reaction you can make target creature do a constitution saving throw, the check is done with disadvantage if the creature has half or less of their health.  If they fail they take 2d8 necrotic damage, if they succeed they take half damage. 

If you cast this spell at a higher level you can add 1d8 to the damage for every spell level above the first.