If you complete the sidequest associated with the Fell Arms (check the FAQ's) they become powered up. For every kill you make with a character their power increases, although it won't show in the stats menu.
This works retroactively as well, so you'll be guaranteed of some strong weapons once you complete the sidequest.
It shows it in the stat increase for me, wierd. Anyways you gotta unseal the fel arms first by completing the sidequest after you beat the game. After you talk to flynn you gotta go back and fight the final boss, I know I made that mistake.
Anyways I beleive there's a sticky topic that tells you the ultimate weapons(not including fel arms). I also beleive it lists armor and such as well.
There are no missable Fell Arms. Yuri - Found from talking to the elder in Myrozo Estelle - Releise (Can't spell...) Hollow Karol - Quoi woods (Must have Level 4 ring) Rita - Found in Tarqaron (Go to the very far right in the puzzle room before the Final boss) Repede - Found in the same room as Rita's fell arm Raven - Keiv Moc Judith - Found in the Aer Kerene (Can't spell either...) near Nordipolica
Actually getting the strength up for the Fell Arms is the hardest part, I've only maxed out Yuri and Rita's. I only plan on maxing Repede, Karol and Flynn's Fell Arms --- Barbatos vows #1 to #999: NO ITEMS EVER!!!