{T}, Sacrifice two other creatures: Any number of target players each lose 2 life and sacrifice a creature. You add {B}{B} and draw a card.
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.
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.
You may target players who can’t sacrifice a creature. Those players still lose 2 life.