Blighter

Blighter is a Rift Guardian, a greatly empowered Herald of Pestilence. He can only be encountered as a random boss in a Nephalem Rift.

In combat, he gains following abilities (and can still use normal melee attack for average Physical damage): Since most of his damage is Poison, having a  may make the otherwise challenging fight much easier.
 * Plague Line: each normal wormhand attack creates not one, but several pools, covering a 60-yard long line. Only used when all other attacks are on cooldown.
 * Plagued and Knockback affixes (Plagued has a reduced cooldown and increased duration, and Knockback is guaranteed to trigger on every melee attack and on venom pool tick). Plagued affix is only used when below 75% Life.
 * Poison Nova: every 5-10 seconds, releases triple bursts of Poison bolts in five directions. Each of them has a chance to knock the player back on hit.
 * Plaguestorm: when at 50% Life or less, every 45 seconds Blighter will plunge its poisoned arm into the ground, like a normal Herald of Pestilence. However, this attack will create many venom pools at once, effectively covering all ground in roughly 40 yards of the boss. This attack may not be interrupted, and while it lasts (roughly 3-6 seconds), the boss is not immobile. This is attack does not knock player back, and is especially deadly for melee fighters.

As of patch 2.2, he also gained two more attacks:
 * Plague Ring: similar to Plaguestorm, but all venom pools will appear in form of circle, 40-50 yards away from the boss, then (3 seconds later) another ring will appear, 20-30 yards away from the boss, effectively forcing player to move closer to the boss. After 3-6 seconds, both rings will despawn. Mostly used when players are too far from the boss.
 * Plague Sweep: similar to Plaguestorm, but venom pools are created everywhere within 60-70 yards, in randomized pattern, creating a vast labyrinth of dangerous zones. Mostly used when players are in melee range.