BacThul is a legendary Runeword created by combining a Bac Rune of Ritual and a Thul Rune of Invocation.
How to Create BacThul
You can create BacThul by adding the legendary Bac Rune of Ritual and the rare Thul Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Bac - Travel 5 meters. (50)
Thul - Invoke the Sorcerer's , inflicting Freeze and Vulnerable onto enemies. (300)
Mystical Frost Nova
Overflow
BacThul can not overflow because the amount of Offering you receive from the Bac Rune exactly fills the required Offering of the Thul Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.