Once the two world's combine into one Ghido tells you about 12 legendary weapons, to obtain the weapons you must find several tablets and bring them to the Sealed Castle (your told where to find them so I'm not going to say exactly where they are, takes the fun out of a game). you can only have one tablet at time so once you have one take it to the castle, and if a weapon sounds strong (Excalibur, Masamune, etc.) take it.
Weapons in the sealed castle: Excalibur, Masamune, Sauske's katana, Assasin's dagger, Holy lance, Giant's axe, Fire lash, Sage staff, Magus rod, Apollo's harp, Gaia bell, and Yoichi's bow.
note: while these are some of the strongest weapons in the game there is one final sword that you can get in the cleft of dimensions; the Ragnarok, this final weapon though is protected by one of the strongest bosses in the game, Shinryu, once defeated you obtain the Knight's ultimate sword.