UrTal is a magic Runeword created by combining a Ur Rune of Ritual and a Tal Rune of Invocation.
How to Create UrTal
You can create UrTal by adding the magic Ur Rune of Ritual and the magic Tal Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Ur - Your Minion or Companion kills an enemy or dies. (10)
Tal - Invoke the Spiritborn's Pestilent Swarm, dealing damage to enemies. (100)
Overflow
UrTal can not overflow because the amount of Offering you receive from the Ur 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.