CirTal is a magic Runeword created by combining a Cir Rune of Ritual and a Tal Rune of Invocation.
How to Create CirTal
You can create CirTal by adding the magic Cir Rune of Ritual and the magic Tal Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Cir - Cast the same non-Channeled Skill 3 times in a row. (50)
Tal - Invoke the Spiritborn's Pestilent Swarm, dealing damage to enemies. (100)
Overflow
CirTal can not overflow because the amount of Offering you receive from the Cir 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.