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.

Abilities that trigger when a creature attacks, blocks, or is blocked will trigger during each combat that turn.