If a creature attacking causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.

Whenever Etali attacks, exile the top card of each player's library, then you may cast any number of spells from among those cards without paying their mana costs.

Equipped creature gets +1/+1. Whenever equipped creature attacks, you may search your library for a basic land card, put it onto the battlefield tapped, then shuffle. Equip {2}

Whenever a nontoken creature you control attacks, create a 1/1 red Goblin creature token that's tapped and attacking. Sacrifice this enchantment: Creatures you control get +0/+3 until end of turn.