Araphath was originally a summon you get from the Northern Cave. His attack did max damage, silenced and dispelled enemies, and gave your party a stat boost.
The original rumor is that if you fight Seraph Sephiroth for 30 minutes, Araphath will replace him.
Sounds like a lie to me. What's the point of getting Araphath materia from the game's final boss, when there's no new game +.