Massacre Wurm is a fun sideboard card for heartless decks, particularly if you are playing any weenie decks. Zero drop spellskites are also useful (but $expensive). Dissipate is a useful sideboard card particularly late in the game. 26 lands seems kind of aggressive to me, I don't know, you may play the deck and find that 24 lands work well. Just my two cents. Heartless decks are really fun to play, but can be extremely challenging if you don't draw a heartless summoning in the opening hand. You may consider adding Think Twice or Forbidden Alchemy to increase your odds of drawing heartless summoning or that Rune Scarred Demon. Good luck.
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