Helm Splitter and Skull Splitter are both Venus class Psynergies in the Golden Sun series, although Skull Splitter was not introduced until Golden Sun: The Lost Age. They are only available to the Samurai and Ronin classes at level 33. Helm Splitter is available at the Samurai class, but upon reaching the Ronin class Helm Splitter is removed from the Adept's available Psynergies when Skull Splitter is added; in other words, Skull Splitter effectively replaces Helm Splitter. Being available to only these two classes, only Isaac, Felix, Garet, and Jenna have access to them.
Helm Splitter: "Paralyze a foe with a mighty blow." Helm Splitter costs 8 Psynergy Points to cast, targets one enemy, and does damage equal to the base attack plus 30 damage, along with a chance to immediately down the foe, instead of stunning them, contrary to what the description says.
Skull Splitter: "Annihilate a foe by crushing the skull." Unlike most Psynergy pairings of similar nature, Skull Splitter has the exact same stats as Helm Splitter, meaning it costs 8 Psynergy Points to cast, targets one enemy, and does damage equal to the base attack plus 30, along with the chance to immediately down a foe. In point of fact, this is completely identical to Helm Splitter save the name and the visual.
Given that this move has a one-hit-kill ability and is relatively cheap, it has the potential to be useful. However, given that the class series also has moves like Quick Strike, which multiplies damage, along with the large attack increase inherent to the Samurai class series and the natural ineffectiveness of one-hit-kill moves against bosses, which is their target of greatest worth, then the overall value of this move is not great. Furthermore, the Annihilation Psynergy of the Ninja class series will do greater damage for the same effect, although the increased PP cost may be a deterrent.