CemTun is a magic Runeword created by combining a Cem Rune of Ritual and a Tun Rune of Invocation.
How to Create CemTun
You can create CemTun by adding the magic Cem Rune of Ritual and the magic Tun Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Cem - Cast Evade. (50)
Tun - Evoke the Rogue's Stun Grenades, Stunning and dealing damage to enemies. (100)
Overflow
CemTun can not overflow because the amount of Offering you receive from the Cem 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.