TamTal is a legendary Runeword created by combining a Tam Rune of Ritual and a Tal Rune of Invocation.
How to Create TamTal
You can create TamTal by adding the legendary Tam Rune of Ritual and the magic Tal Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Tam - Cast a non-Channeled Core Skill. (25)
Tal - Invoke the Spiritborn's Pestilent Swarm, dealing damage to enemies. (100)
Overflow
TamTal can not overflow because the amount of Offering you receive from the Tam 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.