Xenos Cunning is an ability found on cards in the Warhammer 40,000 Commander decks, tied to the Necron faction. When a creature with Xenos Cunning attacks, you may pay an energy counter (one of the resources you've accumulated). If you do, you return that creature to your hand at the beginning of the next end step. This lets you "blink" the attacker back to redeploy it, dodge removal, or re-trigger its enter-the-battlefield effect.
The common mistake is forgetting it requires energy, the counter mechanic—you must already have energy stored to use it, so it synergizes with cards that generate energy. Also note the bounce happens at end step, not immediately, so the creature still deals combat damage that turn.
In Commander, it shines for protecting key creatures from board wipes or sorcery-speed removal, and for abusing strong ETB effects repeatedly each combat.