All-Out Assault

The extra combat is the payoff, but the sequencing is the whole puzzle, and it runs the opposite way from instinct. Read the trigger carefully: the untap fires "when you next attack," meaning when you declare attackers. That is the tell for how this must be played. You cast it in your first main phase and resolve it, and the enchantment hands you an additional combat phase right there; when you declare attackers in that combat, the delayed trigger untaps everything you just committed. Now those creatures are upright and swinging with your whole board rather than a fistful of tapped bystanders. Try to cast it in a second main phase after you have already attacked and the machine sputters: any creatures tapped from the earlier swing stay home, since the untap only fires once you declare attackers, too late to add them to the bonus combat. The main-phase clause on the enter ability is not there to police your own casting (this is a sorcery-speed enchantment with no flash, so you were always casting it on your turn) but to stop blink and cheat-into-play tricks from spawning combats out of turn. Fold deathtouch onto the anthem and every attacker becomes something no blocker profitably eats, which converts a raw extra swing into a race the defender cannot answer. The line that buys a whole extra offensive turn for an entire board, then makes each point of it lethal on contact.