UrThul is a rare Runeword created by combining a Ur Rune of Ritual and a Thul Rune of Invocation.
How to Create UrThul
You can create UrThul by adding the magic Ur Rune of Ritual and the rare Thul Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Ur - Your Minion or Companion kills an enemy or dies. (10)
Thul - Invoke the Sorcerer's , inflicting Freeze and Vulnerable onto enemies. (300)
Mystical Frost Nova
Overflow
UrThul can not overflow because the amount of Offering you receive from the Ur 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.