The Wisest Being - Act 5 Story Quest
The Wisest Being is the sixth quest in Act 5 of Titan Quest: Ragnarok and it begins by speaking with Loki in Valholl immediately after completing The Rescue quest. After speaking with Loki you'll then want to follow Wodan to the Himinbjorg summit (pictured above) where you will want to speak with Heimdall to continue the quest.
Speak with Wodan one last time to open the golden snake gate that blocks your way to the sparkling bridge and then continue northeast across the Bifrost. If your quest bugs out in this area and the gate doesn't open or you can't speak with Heimdall after he blows the horn, try logging out and back in (I mean completely out of the game). If that doesn't work press "K" or whatever button you have bound to the "Reset Player" option under Keybinds while standing as close to the gate as possible, sometimes this will put you on the other side.
Note: While running through the Walls of Utgard you'll pass through Utgard Prison where you'll find another Ylva in a jail cell, releasing her or talking to her is not mandatory to complete the main story. However, speaking with Ylva is part of the hidden quest which involves Loki's Wand. For more information about this quest check out my Loki's Wand Hidden Quest Guide.
Make sure you grab the Portal in the Jotunheimr Wasteland area; you'll reach this zone after crossing the Walls of Utgard area. The Portal is found on the border of the Jotunheimr Wasteland and Hnitbjorg. There will be a small island connected by two land bridges which has a vendor as well as this Portal on it.
Side Quests In Area:
Legendary Craftsmanship (Southern Hnitbjorg)
There is only a single Side Quest for you to do in this area and that's to collect the Mead of Poetry in the southern most portion of Hnitbjorg for the Legendary Craftsmanship quest. Once you've obtained the Mead of Poetry you will want to go far north of the Jotunheim Portal until you find Mimer's Maze.
Mimer's Maze is a nightmare, I would recommend you skip as many enemies in this place as possible. The ghosts in here will strip your buffs off of you and they're very annoying to kill since they are undead. You will find Mimer in the northwestern most portion of his zone.
The fight with Mimer is pretty straight forward, the most difficult part is typically identifying which Mimer is the correct one when he splits into 4. Look at my screenshot above and you will see the real Mimer is a different color than the others. When the split first happens this is the piece of information you'll want to look for to identify the right Mimer.
Additionally, when Mimer splits your DoTs and any debuffs will be on the real one only. So pay attention to which Mimer is ticking from your DoT damage. Once you've defeated Mimer you'll be able to speak with him to complete the quest and begin the followup and final quest in Act 5, The Burning Sword.