Blood Sacrifice

As an action, you draw your ritual blade from your equipment, and make a deep cut across your palm. Choose any number of creatures within range, up to your spellcasting level. Each target must make a Constitution saving throw. On a failed save, the target is marked as a sacrifice with a glowing red sigil, and will forcefully take part in the ritual. On a successful save, the target is not marked, and nothing happens.

Once all targets of the ritual are determined, expend and roll your total hit dice. You sacrifice health equal to the total you rolled, and your hit point maximum is reduced by this amount. You stab the blade deep into your chest, and blood erupts from every orifice of the ritual targets, until they are fully drained of blood, dealing 10d10 necrotic damage + your sacrificed hit points. 

If you are reduced to 0 hit points from this spell, you immediately fall unconscious, and fail your first death saving throw. You cannot use this spell again until your hit point maximum has returned to full, regaining 20 hit points towards your maximum per long rest. (See "Overcasting" rules).

This spell also requires the use of a ritual weapon. The blade must be worth at least 200gp, and be attuned to the user.