Relentless Assault

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

A creature that attacks each turn if able must attack only once that turn. It is not forced into each attack that turn. However, a creature that attacks each combat if able must attack again if it can.

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.