"Mercy? Mercy is for the playground, not the battleground."

Relentless Assault

Untap all creatures that attacked this turn. After this main phase, there is an additional combat phase followed by an additional main phase.

Mystery Booster 2

Commander Legends

Duel Decks: Merfolk vs. Goblins

Planechase

Tenth Edition

A creature that must attack each turn, must only attack once that turn. It is not forced into each attack that turn.

Abilities that trigger when a creature attacks, blocks, or is blocked will trigger during each combat that turn. Thus, a creature with Rampage will get the bonus added during each attack.

If you manage to cast this during a main phase of your opponent's turn, that opponent's creatures will untap and that opponent will be able to attack again. It will not allow you to attack during their turn.

It only creates an additional combat and main phase if it resolves during a main phase.