Unyielding Gatekeeper

A disguise ability lets you cast a card face down by paying 3 generic mana and announcing that you are using a disguise ability. Any time you have priority, you can turn a face-down permanent with disguise face up by paying its disguise cost.

If you controlled the target permanent earlier in the turn but not at the time Unyielding Gatekeeper's last ability resolves, you won't return it to the battlefield tapped. The player who controlled that permanent at the time it was exiled will create a Detective.

The face-down spell has no mana cost and a mana value of 0. When you cast a face-down spell, put it on the stack face down so no other player knows what it is, and pay 3 generic mana to cast it. This is an alternative cost.