Thumping Heart

This spell conjures the vital rhythm of a heart just before death, beating faster and faster. A glassy, reddish heart appears at a point you can see within range; when it appears, it beats once, a dire sound that entreats death.

Each creature within 20 feet of the heart when it beats must succeed on a Constitution saving throw or suffer 1d8 necrotic damage and 1d8 thunder damage. On subsequent turns you can cause the heart to beat again; when you use this action on consecutive turns, the heart beats one additional time to a maximum of three times. When the heart beats more than once on your turn, creatures in the affected area only have to make one saving throw.

If a creature fails the saving throw by 10 or more, their hit point maximum is halved until they finish a long rest. 

If you ever don't use your action to cause the heart to beat, the number of times it beats is reset to 1. If you do not use your action to cause the heart to beat for three consecutive turns, it withers and the spell ends.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage caused by the heart beats increases by 1d8 (both the necrotic and thunder) for every two slots above 3rd.