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