25 Jan 2019
Because it may have targets, the ability of Priest of Forgotten Gods isn't a mana ability. It uses the stack and can be responded to, even if no targets were chosen.
25 Jan 2019
You can activate the ability of Priest of Forgotten Gods without choosing any targets if you wish. You'll still add {B}{B} and draw a card.
25 Jan 2019
You may target players who can't sacrifice a creature. Those players still lose 2 life.