UrZec is a rare Runeword created by combining a Ur Rune of Ritual and a Zec Rune of Invocation.
How to Create UrZec
You can create UrZec by adding the magic Ur Rune of Ritual and the rare Zec 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)
Zec - Reduce your active Ultimate Cooldown by 2 seconds. (200)
Overflow
UrZec can not overflow because the amount of Offering you receive from the Ur Rune exactly fills the required Offering of the Zec Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.