Merciless Resolve — ruling 1

You must sacrifice exactly one creature or land to cast Merciless Resolve. You can’t cast it without sacrificing a permanent, and you can’t sacrifice additional permanents.

Merciless Resolve — ruling 2

Players can respond to this spell only after it’s been cast and all its costs have been paid. No one can try to destroy the creature or land you sacrificed to stop you from casting this spell or to make you sacrifice a different one.

Merciless Resolve : règles, fonctionnement et interactions (MTG)

Merciless Resolve 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

As an additional cost to cast this spell, sacrifice a creature or land. Draw two cards.

How it works : Merciless Resolve

Original English text — Wizards of the Coast / Scryfall

#1April 8, 2016· Wizards of the Coast
You must sacrifice exactly one creature or land to cast Merciless Resolve. You can’t cast it without sacrificing a permanent, and you can’t sacrifice additional permanents.
No other card is named in this ruling.
#2April 8, 2016· Wizards of the Coast
Players can respond to this spell only after it’s been cast and all its costs have been paid. No one can try to destroy the creature or land you sacrificed to stop you from casting this spell or to make you sacrifice a different one.
No other card is named in this ruling.