Talk to Councillor Jake. He will tell you that it was the White Witch who killed him.
Go to Luculla Forest entrance by heading northeast from Blossius' mill from The Legionnaire's Will. Proceed to Luculla Forest via the wooden bridge.
Head northwest from the Cyseal Road Waypoint you've just unlocked. You will come across a fleeing Immaculate named Samid who will start a dialogue with you. Exhaust all options to receive Infiltrating The Immaculates and a quest marker revealing the location of the Witch's cabin. Go there now.
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 remove the barrier using whichever method you chose.
Go inside and use the mirror to teleport to a cave. Proceed until you meet Vaelanna. Kill her and loot Vaelanna's Orders. Read the Orders or the Cultist Spell in front of the magical lake to turn the lake into a portal to the faery world of Hiberheim.
Enter the portal. You'll find Enchanted Sentinels which will crumble as you walk past 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.
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. You will be ambushed by seven Mecha-Rodents (Beta) when you open the chest.
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 wolves. Continue east to find the Hiberheim Forest Waypoint Portal. Directly to the south of the portal, destroy the ice encasing Arroka to start the quest The Captives in the Crystals.
From the portal, follow the path north and continue north on the path between the large head shaped boulder and the immaculate camp. When the path forks, take the northwestern path until you come across another large, skull shaped boulder. 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 them, but will still be where you last saw them. Your companions can step on the invisible traps so it is best to send one character in alone and use the Teleporter Pyramids to regroup 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 get past them. Use the Hiberheim Prison Waypoint Shrine to regroup. Proceed until you find the White Witch imprisoned in a giant ice crystal.
Teleport to the Hiberheim Castle Waypoint to leave the prison (or if you don't have it, head further north from the skull shaped boulder where you entered the prison) and then go east where you'll find several elementals locked in battle. Defeat them and continue to the northeastern-most part of the map until you run into the cultists that guard 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 the Conduit making a Blood Stone. After exhausting all dialogue options, she will summon four demons and then disappear. Kill the demons and one of them will drop a parchment called Orders.
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 in you getting attacked by Ice Elementals but they shouldn't pose too much of the problem. Click on the barrier blocking the entrance and use the Orders you picked up in the forge to disable it. Go inside the castle and kill Boreas. Pick up the Elemental Staff he drops.
Go back to the Elemental Forge and drag the Elemental Staff from your inventory onto the forge to free the other Elemental Kings. Sua, the Fire King, will give you a Fire Rune which you can use to summon him.
Go back to the ice crystal where the White Witch is imprisoned. Use the Fire Rune to summon the Fire King. He will melt the ice crystal and free the White Witch, completing the quest.