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