CemThul is a rare Runeword created by combining a Cem Rune of Ritual and a Thul Rune of Invocation.
How to Create CemThul
You can create CemThul by adding the magic Cem Rune of Ritual and the rare Thul Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Cem - Cast Evade. (75)
Thul - Invoke the Sorcerer's , inflicting Freeze and Vulnerable onto enemies. (300)
Mystical Frost Nova
Overflow
CemThul can not overflow because the amount of Offering you receive from the Cem 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.