NocTal is a rare Runeword created by combining a Noc Rune of Ritual and a Tal Rune of Invocation.
How to Create NocTal
You can create NocTal by adding the rare Noc Rune of Ritual and the magic Tal 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)
Tal - Evoke the Spiritborn's Pestilent Swarm, dealing damage to enemies. (100)
Overflow
NocTal can not overflow because the amount of Offering you receive from the Noc Rune exactly fills the required Offering of the Tal Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.