Raiziel

Raiziel is a Rift Guardian, a greatly empowered Exarch. He can only be encountered as a random boss in a Nephalem Rift.

In combat, he gains increased damage, as well as following abilities:
 * Electrified affix. The more Life he loses, the more sparks he releases per hit.
 * Lightning Orb: the standard attack of Exarch, causes more damage and lasts longer. Strikes faster than normal Orbs of regular Exarchs, and each strike creates an Electrified spark (charged bolt), sending it in random direction. After losing 25% Life or more, casts it more frequently.
 * Nova: releases a radial volley of Charged Construct bolts, which deal Holy damage instead of Lightning.
 * Lightning Touch: the standard melee attack of Exarch, except for it causes more damage and is used more often.

As major Raiziel's attacks are Lightning-based, Xephirian Amulet makes this Rift Guardian much easier.

Trivia

 * As of 2.4, it is the first monster in game able to directly use Holy damage against players.