In the Unknown Palace you will find the most dangerous enemies found in the Soul of Rebirth.
Chaos riders will probably cut your characters health in half or even kill them instantly, but they drop the most powerful axe in the game: the Rune Axe. The only way to be strong enough to take them is usually to use berserk on your party, then its still an uphill battle. The unknown palace is almost exactly like pandaemonium except for different enemies and the chests and the place is practically turned backwards. But if you survive your way to the 4th floor you will have a choice of 4 doors. The farthest one on the left will take you to the 6th floor. the farthest door on the left will lead you to 2 of most powerful weapons in the game. The first weapon is the powerful wyvern lance guarded by a easy enough black dragon which the easiest way to kill it is to use berserk on your party. Guessing by the name you should already know that Richard is the only one who is able to wield the powerful weapon. The other weapon is like how you found masamune in the original game except for 2 very important pieces of information. The first is that it is not masamune it is wild rose which is only wieldable by Scott. Second is that it is guarded by Lucifer a rank 7 badass. He can cause as much mayhem as 2 chaos riders mixed together. the only reason in was able to beat him the first time is because he kept using his magic instead of his melee attacks. you can also find a stardust rod that Minwu can use in a chest in one of the 4 doors.