The Spellsplinter Maneuver is a fighting technique that allows one to interrupt a caster's spell, even if the caster is casting defensively. In the present day, the only known living fighter known to know the maneuver is Roy Greenhilt, who was taught the move by his grandfather Horace in Paradise. Roy has used the move against three opponents: Against an illusionary Xykon while under the power of Girard Draketooth's illusion in his pyramid, Miron Shewdanker while fighting the Vector Legion and the Empire of Blood and against the High Priest of Hel at the Northern Godsmoot. The last use, which was the first time he used it publicly, greatly surprised those assembled at the Godsmoot given that they believed nobody who knew how to use it was still alive.
The maneuver is functionally very similar to the Mage Slayer feat introduced in the 2004 D&D 3.5 supplement, Complete Arcane. It may simply be a campaign-specific name given to this feat. However, the surprise exhibited by the various spellcasters when Roy used the maneuver suggests they were not aware that they could not cast defensively, which they would be if Roy were using that feat as written.