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