Doom seems quite efficient compared to dealing damage directly - 13 effective damage for 1 energy is very high, but you must account for the delayed effect and block for an additional turn to use it properly.
Osty deals 4 damage. Whenever you play a card that costs or more, return this to your Hand from the Discard Pile.
Retain. If Osty is alive, he dies and you gain Block equal to double his Max HP.
Apply 13 Doom. Draw 1 card.
Deal 9 damage. Add Ethereal to a card in your Hand.
Ethereal. Transform a card in your Draw Pile into Soul.
