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