Hamelin

Hamelin is a Rift Guardian, added in patch 2.2.0 in Diablo III. He can only be encountered as a random boss in a Nephalem Rift, and is a colossal Rat King.

In-game
Unlike other Rift Guardians, Hamelin will be inert when spawned, remaining dug into ground until players approach him. When they do, he will spawn a horde of Plagued Vermin and unburrow. In combat, he gains following abilities:
 * Basic Attack, which delivers very fast melee blows for significant damage.
 * Rat Tornado: creates two small (10 yards radius) tornadoes of rats which rapidly and erratically move around, dealing Poison damage to enemies they touch. They do not home at players, but are too unpredictable to avoid them at close range. After roughly 6 seconds, each tornado explodes, dealing Poison damage within 10 yards.
 * Ratlings: every few seconds, Hamelin spawns 5 Plagued Vermin from the holes on his back. There is no limit of how many he can spawn at a time, but rats are very weak and fragile.
 * Plagued Arena: creates a 45-yard radius green circle around the Rat King that is impassable to both players and monsters. The circle lasts 10 seconds and has a 15 seconds cooldown. One may bypass the barrier with any effect that allows walking through Waller affix obstacles.
 * Digger: every 30 seconds, Hamelin may dig into the ground and instanly emerge at another location. Often used if players somehow leave the Plagued Arena.

Development
Originally, Hamelin used to have a teleport attack (this was replaced by the burrow attack with knockback) and upon death, a whole nest of rats would erupt from his corpse to attack the player (in the final version, there is only an animation of this effect).

Trivia

 * The name and concept of this monster are possibly based on the classic Pied Piper of Hamelin fairy tale.


 * He is the only Rift Guardian who gained a boss or normal monster variant after it appeared as a Guardian, and not vice versa.