UrTun is a magic Runeword created by combining a Ur Rune of Ritual and a Tun Rune of Invocation.
How to Create UrTun
You can create UrTun by adding the magic Ur Rune of Ritual and the magic Tun Rune of Invocation to a two-socketed 
Helm, 
Chest Armor, 
Pants, or 
Two-Handed Weapon.
Overflow
UrTun can not overflow because the amount of Offering you receive from the Ur Rune exactly fills the required Offering of the Tun Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.
Legions
Helltide