Divinity: Original Sin - Enhanced Edition

Story related so you won't be missing this one. For those who want to know when and where this unlocks, read on.

Once you have completed "Forged in Fire" by releasing the elements in Boreas' Staff, you will meet the fire elemental "Sua". In return for freeing him, he will offer his assistance in freeing the White Witch from her Icy Prison and give you a rune to call him to the location when you are nearby.

If you haven't already found the prison, there is a dig spot to the south east of the Castle Waypoint, you'll need good perception to see it. Dig and go down into the cave, through to the other side and up a ladder to bring you into the prison.

Once you have navigated the various traps and trials in the prison, at the end of the linear area, you'll find the White Witch in ice. Call the elemental Sua by using the rune whilst standing near the ice and he will release her. Unlocking Free the Witch!