A legendary blade awaits you in Fenrir's Cavern, which is found in a church in Freezekill.
Prepare for Fenrir's Cavern
Make sure that you have at least five Rune Bottles and at least two Mystical Runes. Have Arche use only her fire elemental spells, and turn off Klarth/Claus's Undine, Sylph, and Gnome. You will want Cless/Cress to use his fire elemental skills, like Phoenix and Magma Rift.
Go into Freezekill (which you can reach by flying south from the Lake of Ymir and then southwest from Ary) and use the northern exit to find a church. Check the front door of the church to open it. Go in. Go north, then after the cutscene, walk behind the pulpit to open the door. Go in.
Explore Fenrir's Cavern
Go down the stairs. Open the chest to get a ?Helm, which becomes a Paladin Helm. Then equip the Sorcerer's Ring and shoot the switch on the wall. Check the switch, then flip it. Go east.
Go north, through the door, then down the stairs.
Use the Sorcerer's Ring to light the two candles, then go east and light those candles, then go south and follow the path to light three sets of candles. Go back to where the path splits and go east to find the last pair of candles. From there, go south, go through the door, open the chest to get an ?Armor that becomes a Star Cloak, then go down the stairs.
Follow the path until you reach a statue. Use the Sorcerer's Ring on it, then push it north, not west or east. Push it as far north as it will go. It will stop in front of the hole in the wall. Then go east and light the candles. Southeast from the candles is part of a wall. The door is in that wall. Go south through it and go down the stairs.
Follow the path, and when you see water, go west.
It is worthwhile to get all of the treasure in this maze. But if you would rather not get it right now, then from the four-way screen that you start in, just go west twice, where you will find a Black Onyx, then go north until you are out of the maze.
If you don't want all of the treasure, but you do want a new spell for Arche, then from the start of the maze, go south and you will find an Armlet, then go west three times and you will find the Tidal Wave spell (which you should turn off for now), then go east, then north, where you will find a Black Onyx, then just keep going north.
If you do want to get all the treasure, then from the start of the maze, go south and you will find an Armlet, then go west and south, and you will find a Charm Bottle, then go north, then west twice and you will find the Tidal Wave spell (which you should turn off for now), then go east and north, where you find a Black Onyx, then go west and you will find a Silver Cape, then go east, then north and east to find an Aqua Mantle, then go north, then east twice to find a ?Book that becomes Ex Mortis, then go west and north and you will find a Silver Plate, then go south, then west until you find a Shield Ring, then go east and north, where you will find a room with stairs. Go down the stairs.
From the stairs, go south. If you don't have the Rare Shield, west and go through the leftmost door and get the Ankh Shield. Go back through the door, then go through the door to the right of the one that you just came through. Your path will be blocked because there is a chest north of you. Open that chest to get a ?Helm, which becomes a Star Cap. After you get the ?Helm, go north, then east to the next area.
The path goes in a spiral toward the center of the room. At the center, you find a switch. Flip it. Then go to the north part of the room and go east to the next area.
The path through this area is straightforward, and eventually you reach an open door. Go through it and flip the switch, but don't go through that door yet. Instead, go east to the next area.
Go north through the door and open the chests. The one on the left contains a ?Bow that becomes Gale Spread (which is not better than the Elven Bow), and the one on the right contains Cute Mittens. Go back through the door, then go west.
Keep going west until you see an open door, and go through it. Step on the switch, then exit and go down the stairs to the left, onto the frozen water, and follow the path to the next area.
Go up the stairs and through the door, then go through the door directly to the left, then follow the path east to the next area.
Go around the spiral to the middle of the room and flip the switch, then, as before, go to the north of the room and go east to the next area.
Walk onto the ice, then go east and there will be a piece of the floor that you can walk onto to exit north.
You will reach a save point. There will be a battle soon, so take some time now to prepare. Set Klarth/Claus and Arche's tactics to not cast spells. Set Cless/Cress to use Magma Rift and Phoenix. Equip him with a non-elemental weapon like the Mecha-Halberd. Equip Klarth/Claus and Arche with Mystical Runes, and also Mint if you have an extra one. Equip Klarth/Claus with the Aquamarine Ring. Equip the Aqua Mantles on Cless/Cress and Mint. Equip Arche with the Dark Robe, since it blocks many elements. Equip Cless/Cress with an Armlet because he is likely to be stunned often. For the formation, put everyone on the leftmost side.
Go north to the next area. You won't be attacked in the room with the columns, so if you need to heal up, fill your food sack and walk around the entrance until your health is restored. Use Orange Gummies on anyone whose TP is low.
FangWolf has a spear attack and a punching attack that do high damage to anyone near him, so stay far away from him at all times, and attack as quickly as possible to try to end the battle before FangWolf gets too close.
Have Cless/Cress use Phoenix throughout the entire battle. Have Mint use Hammer on Fenrir repeatedly. Arche and Klarth/Claus should be casting Fire Storm and Maxwell as quickly as possible.
Fenrir's Cavern, Part 2
After the cutscene, you get the Vorpal Sword and a new long range skill: Soul Edge. Assign Soul Edge to Cless/Cress to allow him to master it as soon as possible. However, do not equip the Vorpal Sword yet, because it heals the monsters in this area. Magma Rift works well as his close range spell.
Make sure that Klarth/Claus's spells are disabled except for Ifrit.
Change your formation back to normal.
To exit the cave from the save point, go south, then west across the ice to the next area, then use the west exit of the room with the switch in the center, then go as far west as you can go and then go south through door there, then go up the stairs to the right of the door that you came out of, then follow the path until it splits, then go south twice and east three times, then go south and follow the path and go up the stairs at the end, then go north through the door, then go west and follow the path to reach the stairs, and go up them, then go north through this door, then go all the way west, then go up the stairs, then go west and south to the next area, then follow the path until you reach some stairs, and go up the stairs and go through the door at the top, then save if you wish, and go southwest to exit the cavern.
Your next destination is Odin's Tower, which is in the mountains of the Freland continent, surrounded by lava, near Ifrit's cave. It is south and slightly east of Freezekill.