Not incredible damage per energy ratio, but putting it only a 3-Cost card with Retain ensures that you can hit enemies decently well on turns where they don't attack you, and Necrobinder has some synergies with high-cost Cards and high-damage Attacks.
Sometimes okay to grab for Act 1 damage.
Tons of damage for one card draw, and the Retain effect means you can keep this in hand until it finishes the fight or until you have spare energy to use on it.
Osty deals 7 damage. Hits an additional time for each other time he has attacked this turn.
Summon 20. Exhaust.
Retain. Deal 27 damage.
Whenever Attacks deal damage, they also apply that much Doom.
