"We will give you the peace you seek, though you may not yet know you seek it."

Mercy Killing

Target creature's controller sacrifices it, then creates X 1/1 green and white Elf Warrior creature tokens, where X is that creature's power.

The List

Shadowmoor

Target creature's controller sacrifices it, then puts X 1/1 green and white Elf Warrior creature tokens into play, where X is that creature's power.

You target the creature that will be sacrificed when you cast Mercy Killing, then its controller (who may or may not be you) sacrifices it when Mercy Killing resolves. Because this is a sacrifice, neither regeneration nor indestructible will save the creature.