UrXal is a rare Runeword created by combining a Ur Rune of Ritual and a Xal Rune of Invocation.
How to Create UrXal
You can create UrXal by adding the magic Ur Rune of Ritual and the rare Xal Rune of Invocation to a two-socketed 
Helm, 
Chest Armor, 
Pants, or 
Two-Handed Weapon.
Overflow
UrXal can not overflow because the amount of Offering you receive from the Ur Rune exactly fills the required Offering of the Xal 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