Zantetsuken is a rare and deadly attack from various Final Fantasy games. I've seen it in Final Fantasy VI, Final Fantasy VII, and Final Fantasy VIII, and I would imagine it's in a few of the ones I haven't played.
Zantetsuken has been translated as either 'Iron-Cutting Sword', which is a direct translation of the kanji in the word, or 'Steel-Bladed Sword', as steel, being stronger than iron, probably would be able to cut it if wielded by a sufficiently powerful warrior.
Zantetsuken, as far as I have seen, is always the attack of the summonable Esper/Materia/Guardian Force Odin, and, if the attack is successful, Odin simply slices the monster in two, rendering him inanimate.
In Final Fantasy VIII, at some point in the game Gilgamesh
arrives and cleaves Odin himself (it was much to my shock and dismay when I witnessed this, the demise of my favorite invincible magical horseman of death!), and replaces him, giving you a random one of four attacks when he appears, one of which is Zantetsuken.