Talk to Councillor Jake. He will tell you about a certain White Witch who killed him.
Go to Luculla Forest entrance by heading northeast from the Blossius' mill from The Legionnaire's Will. Proceed to Luculla Forest via the wooden bridge.
Head southeast from the Cyseal Road Waypoint. You will come across a fleeing Immaculate named Samid who will start a dialogue with you. Exhaust all options to receive the quest, Infiltrating The Immaculates and a quest marker revealing the location of the Witch's cabin.
To deactivate the barrier around the cabin, speak to the talking mushroom Icari next to the cabin to get the quest Fun With Fungi. Finish this quest and read the Barrier Removal Spell to remove the barrier.
Go inside and use the mirror portal to teleport to a cave. Proceed until you meet Vaelanna. Fight with her and loot Vaelanna's Orders. Read the Orders or the Cultist Spell in front of the magical lake to make the lake become a portal to the faery world Hiberheim.
Enter the portal. You'll find Enchanted Sentinels, which will crumble as you walk pass them. Follow them until you see a stone spike gate blocking the path. Step on a pressure plate on the rocky part of the floor in front of the gate to open it. Send another character to the other side and use the Teleporter Pyramids to regroup your party.
Directly south of the gate there is a lever you can use to permanently open it. You will be attacked by two Mecha-Rodents (Alpha) and four Mecha-Rodents (Beta).
Follow the path northwest until you reach a fork in the path. Take the path to the northwest, but stay on the least elevated part of that path. Here you will find a hidden mound that contains a key. Continue northwest and unlock the lever to open another gate. You will be attacked by two Mecha-Roosters. After you defeat them, follow the path until you meet the Witch's apprentice, Almina. She gives you the quest, Eternal Winter and then dies.
Now, backtrack to the gate from step 5. Take the path south until you reach a Watchful Sentinel and continuously falling ice shards that prevent you from continuing. There is a pressure plate directly south of the Watchful Sentinel which stops the ice shards. Use the same technique from step 5 to cross it.
The path instantly forks to the north and the east. Be careful of mines. To the north there is an optional loot chest. Opening the loot chest will cause you to be attacked by seven Mecha-Rodents (Beta).
Continue down the eastern path and go southeast up a hill. Remove the barricade and then use the same technique from Step 5 with the pressure plate atop the hill to pass the gate. There is a lever directly north of the gate that will permanently open it.
Continue east along that path and you will engage a pack of five wolves. Continue east to find the Hiberheim Forest Waypoint. Directly to the south of the Waypoint, destroy the ice encasing Arroka to start the quest The Captives in the Crystals.
From the Waypoint, follow the path north, passing the first large boulder in the shape of a head on east and an Immaculate camp on the west side of the path. When the path forks, take the northwestern path until you come across another stone skull. Approach it and did the snow mound directly in front of it to reveal a hidden hatch. Use the hidden hatch to continue to the next area.
Continue northeast down the cave, but be wary of the mines in the fog of war. They will become invisible as you approach, but will still exist where you last saw them. Your companions can step on the invisible traps so it is best to send one character and use the Teleporter Pyramids to reunite the party on the other side. Proceed northeast down either path until you find a ladder. Use the ladder to climb up to a prison cell.
Dig the mound in the east corner of the cell to enter the next cell, which is not locked. Leave the cell and destroy the Iron Maiden to the north to get a Blood Stone.
Now continue southeast until you reach a room with four Watchful Sentinels. Use invisibility or sneaking to bypass them. Use the Hiberheim Prison Waypoint Shrine to regroup your party. Proceed until you find the White Witch imprisoned in a giant ice crystal.
Use the Hiberheim Castle Waypoint to leave the prison and then go east where elementals are fighting with each other. If you do not have that Waypoint, then use the Hiberheim Forest Waypoint and follow the path northeast. Defeat them and continue to the most northeastern part of the map until you get attacked by cultists guarding the doors to the Elemental Forge. Kill the cultists and go through the doors.
Continue northeast down the stairs. Talk to Jijix and exhaust all dialogue options. Continue northeast and you will meet a Conduit making a Blood Stone. After exhausting all dialogue options, she will summon four demons and then flee. Kill the demons and one of them will drop the Orders. Pick up the paper.
Use the Hiberheim Castle Waypoint and go northwest into the castle of Ice King Boreas. Walking into the vision of the Watchful Sentinels on the bridge will result you being attacked by Ice Elementals. You can choose to avoid this if you have either 7 sneaking or invisibility. Click on the barrier blocking the entrance and use the Orders to remove it. Go inside the castle and kill Boreas. Make sure you take the Elemental Staff which Boreas drops.
Go back to the Elemental Forge and smelt Elemental Staff at the Forge to free the other Elemental Kings. Sua the Fire King will give you a Fire Rune.
Go back to the ice crystal where the White Witch is captured. Use Fire Rune to summon Fire King Sua. He will melt the ice crystal and free the White Witch.