Denathrius is fine, Kael'thas is not fine. Nerf Kael'thas to not count itself for the 3 minions.
Impending catasthrope to 3 mana at least.
Change Guff hero power to give empty mana crystals.
Fix double Collateral Damage bug.
Maybe change Serpent Wig to +1/+1 or Radiant Elemental to 3 mana...
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^this. i see u can be a little bit meh if you only play HS, but seen from a world of warcraft viewpoint, this card makes great sense and is very nice design/theme-wise.