Lord of Bells is a Rift Guardian that will only spawn at the end of the very rare Infernal Bovine Rift in Diablo III.
For those who missed Lightning Enchanted Cow King
Like his predecessor, Lord of Bells has empowered Electrified affix: on every hit it releases very many sparks, just like it did in the second game. The amount of sparks grows even further as the boss loses more and more Life. In addition, it also has the Illusionist affix, and periodically summons a horde of ghostly bovine guards to his (her?) side.
Originally, this boss was called the Cow Princess, Heir to the Herd.[1]
As of patch 2.1.2, killing Lord of Bells is no longer required for the "You're Not The Boss Of Me" meta-achievement. Prior to patch 2.4, it also could use the Threatening Shout skill.
Upon death, Lord of Bells is guaranteed to drop Man Prodder.
The Cow Princess[]
The Cow Princess, Heir to the Herd, has been added again in patch 2.1.1. The abilities and appearance of this boss are exactly the same as those of Lord of Bells, plus, she has Thunderstorm affix. In return, this boss only spawns if players touch the cursed jar (very similar to the Jar of Souls) during the mini-event called Udder Chaos.
Cow King during the Udder Chaos event
She does not count as a Rift Guardian (so the Rift will not end with her death). She does, however, drop loot like a Rift Guardian, even in a Greater Rift. She does not spawn as a normal Rift Guardian upon filling up the Rift progress bar.
This event may only occur in a layout identical to the Defiled Crypt. Ghost of the Cow King will taunt the player after the fight.