Shipwreck Singer

Flying
{1}{U}: Target creature an opponent controls attacks this turn if able.
{1}{B}, {T}: Attacking creatures get -1/-1 until end of turn.

The List

Conspiracy: Take the Crown

Her melody melds death and beauty with such artistry that even the gods weep to hear it.

Theros

If, during a player's declare attackers step, a creature is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under that player's control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so it doesn't have to attack in that case either.

The controller of each attacking creature still chooses which player or planeswalker that creature attacks.

The last ability affects creatures that are already attacking when it resolves. Activating it before combat won't affect any creatures that attack later that turn.