Detonate Undead

You mark 2 Undead Creatures you control within range (60 ft) with a necromantic channeling rune. As an Action, you can chose to detonate a single rune within a mile of you, causing an Undead creature you control within range to be overfilled with necromantic energy, destroying it and sending out a blast of necrotic energy. Every creature in a 20ft. radius sphere centered on that undead must make a Dexterity Saving throw or take 6d6 Necrotic damage on a failed save, or half as much on a successful one. The blast spreads around corners. Undead that are marked with the rune detonate when they are reduced to 0 HP. This runes stay for 1 week or until this spell is cast again

At Higher levels: When you cast this spell using a spell slot of 4th level or higher, the number of runes increases by 1 for each slot level above 3rd.