Blastbones

This spell must be cast on an exposed skeleton. It cannot be cast on the bones of a living creature (assuming they are still attached to the creature) but may be cast on an undead skeleton. The targeted set of bones explodes, dealing 2d4 + 2 piercing damage to creatures within a 10-foot radius of it. When you cast this spell, you can choose whether to detonate the bones immediately or delay the blast for up to one hour. If you choose to delay the blast, then you can set certain conditions (“Whenever that door is opened”) or a command word that will cause the bones to explode.