Detonate Bones

Choose up to 4 undead creatures that are similar to skeletons and are of CR 1/2 or lower. If they are friendly to you, their bones will implode and then immediately detonate, sending shards flying in all directions. Any creature within 10 ft of a skeleton must make a Dexterity saving throw against your spell DC and take 1d6 piercing damage on a failed saving throw, or half as much on a success. One creature may be affected by more than one targeted undead but only makes a saving throw once for all targets. Creatures surrounded by targeted undead have disadvantage on the saving throw.

If you target an undead that is not friendly to you, it must have no more than 9 hit points or the spell has no effect.