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