ZanThul is a rare Runeword created by combining a Zan Rune of Ritual and a Thul Rune of Invocation.
How to Create ZanThul
You can create ZanThul by adding the magic Zan Rune of Ritual and the rare Thul Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Zan - Cast an Ultimate Skill. (150)
Thul - Invoke the Sorcerer's , inflicting Freeze and Vulnerable onto enemies. (300)
Mystical Frost Nova
Overflow
ZanThul can not overflow because the amount of Offering you receive from the Zan 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.