My personal Umbral Nekros build, focused around turning him into a veritable tank with wide area crowd-control thanks to Gloom.

Gloom is intended to be the first line of defense for this build, since enemies in the aura are slowed substantially. While it does drain energy, the high energy pool from [Primed Flow] as well as the mitigation of energy cost from [Despoil] make it so that the only two abilities that will cost you energy at any given time will be Gloom and Shadows of the Dead.

Gloom also provides a modicum of tankiness with its lifesteal, though the bulk of [Nekros]' durability in this build (despite it being more of a [Failsafe] in case Gloom should ever drop) comes from stacking high damage reduction values; gaining high armor from Arcane [Guardian] alongside the damage reduction bonuses from [Shield Of Shadows] and [Adaptation].

You should always make sure to try and cast Gloom after proccing [Growing Power], since the only other Ability Strength mod in the build is [Umbral Intensify], so [Growing Power] should help bump up the lifesteal and slow field of Gloom.