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