Make Kyristan's Malevolent Tentacles do Entropy damage with Epic Fathomless pact the same way that it does with Epic Spell Focus Necromancy, since warlocks cannot take epic spell focuses.
Fathomless Warlock is supposed to be THE tentacle AOE spell class, so having Kyristan's be on par with a wizard who took ESF Necromancy would make a lot of sense, and make Fathomless a little better in high levels.