What happens to an Equipment if the creature it's attached to leaves the battlefield?

When a creature leaves the battlefield, any Equipment attached to it becomes unattached and stays on the battlefield under its owner's control. This is a state-based action defined in CR 704.5p, which checks for Equipment attached to something illegal and unattaches it. The Equipment itself is not 'sacrificed' or destroyed — it simply loses its host and sits on the battlefield as a plain artifact.

This happens because Equipment uses the 'attach' mechanic (CR 301.5), and the attachment relationship only exists while both the Equipment and the creature are on the battlefield together. Once the creature is gone, there is nothing to be attached to, so the game immediately severs that link the next time state-based actions are checked.

The Equipment will remain on the battlefield until something else removes it (a destroy effect, bounce spell, sacrifice, etc.). Your opponent cannot target or destroy the Equipment simply because the creature died — they would need a separate effect that hits artifacts.

Concrete example: You have a Longsword (a generic Equipment) attached to your 2/2 Knight. Your opponent casts Lightning Bolt, destroying the Knight. The Knight goes to the graveyard, but the Longsword stays on the battlefield unattached. On your next turn, you can pay its Equip cost to attach it to another creature you control.

Note: If the Equipment has a triggered ability that triggers when it becomes unattached (some Equipment do), that trigger would go on the stack at the appropriate time per CR 603.6. But by default, the Equipment simply remains in play, available to be re-equipped.