KaaThul is a rare Runeword created by combining a Kaa Rune of Ritual and a Thul Rune of Invocation.
How to Create KaaThul
You can create KaaThul by adding the rare Kaa Rune of Ritual and the rare Thul Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Kaa - Lose 25% of your Maximum Life. (50)
Thul - Invoke the Sorcerer's , inflicting Freeze and Vulnerable onto enemies. (300)
Mystical Frost Nova
Overflow
KaaThul can not overflow because the amount of Offering you receive from the Kaa 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.