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