One of the proposed mechanics for the Black Knights Faction from the Goblin Artisans Black Knights design challenge.

This is Aura's Sabotage mechanic.

This might get rejected from a real set for causing problems on MTGO. It gives itself away if I cast a creature and suddenly the game waits asking you something.

An alternative phrasing that still has that trouble:

> Sabotage creatures — Whenever a creature enters the battlefield under your opponent's control, you may cast ~ from your hand by paying rather than its mana cost.

An alternative phrasing to avoid that trouble:

> Sabotage creatures — You may cast ~ during an opponent's main phase for if that opponent had a creature enter the battlefield under his or her control this turn.