Mercy Killing — ruling 1

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.

Mercy Killing : règles, fonctionnement et interactions (MTG)

Mercy Killing is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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.

How it works : Mercy Killing

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
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.
No other card is named in this ruling.