Enchant creature
Enchanted creature gets +3/+3 and attacks each combat if able.
When enchanted creature dies, Impending Doom deals 3 damage to that creature's controller.
If the enchanted creature can’t attack for any reason (such as being tapped or having come under that player’s control that turn), then it doesn’t attack. If there’s a cost associated with having it attack, the player isn’t forced to pay that cost, so it doesn’t have to attack in that case either.