Melancholy

Melancholy is an uncommon Necrobinder skill costing 3 Energy that gains 13 Block and permanently reduces its cost by 1 whenever ANYONE dies, including enemies, minions, and even Osty. Over the course of a run, Melancholy's cost drops as creatures perish, eventually becoming a free or cheap massive Block source.

Melancholy's cost reduction triggers from Bone Shards and Sacrifice killing Osty, from Misery and End of Days killing enemies, and from general combat attrition. Death's Door provides additional Block that pairs with Melancholy's large Block value. Delay offers supplemental Block and Energy generation for turns where Melancholy is still expensive.

In any Necrobinder archetype, Melancholy improves over the course of a run as its cost decreases. Against bosses with summons like Doormaker and Ceremonial Beast, minion kills rapidly reduce Melancholy's cost. In Osty/Summon builds where Bone Shards or Sacrifice regularly kill Osty, the death trigger fires frequently. Early in a run it is clunky at 3 cost, but by late bosses it can provide 13 Block for 0-1 Energy.