NocTun is a rare Runeword created by combining a Noc Rune of Ritual and a Tun Rune of Invocation.
How to Create NocTun
You can create NocTun by adding the rare Noc Rune of Ritual and the magic Tun Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Noc - Inflict a Crowd Control that isn't Slow or Chill. (5)
Tun - Evoke the Rogue's Stun Grenades, Stunning and dealing damage to enemies. (100)
Overflow
NocTun can not overflow because the amount of Offering you receive from the Noc 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.