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