Sidequest: Hunt the Mark

In the Commander variant, a double-faced card's color identity is determined by the mana costs and mana symbols in the rules text of both faces combined. If either face has a color indicator or basic land type, those are also considered. For example, Cecil, Dark Knight's color identity is black and white, since its front face is black and its back face has a white color indicator.

Sidequest: Hunt the Mark's last ability checks at the moment it would trigger to see if a creature died under an opponent's control this turn. If none did, the ability won't trigger at all. Once your end step begins, it's too late to cause creatures your opponents control to die in order to cause this ability to trigger.

The mana value of a nonmodal double-faced card is the mana value of its front face, no matter which face is up.