STRENGTH: We use Transient Fortitude, Vome Invocation on our secondary, and an archon shard. With this we'll get to 170% ability strength from the get-go. With Sling Strength we get a 73.5% slow from Gloom and a 94.1% damage reduction on your shields and 91.86% damage reduction on your health from Shadows of the Dead which we actually don't care much about since this build is more focused on shield gating than tanking. If we wanted to tank we can change some mods and arcanes we'll mention on the survivavility section.
If we find enough enemies a single alt fire from our [Grimoire] will give us +60% ability strength. That along with Sling Strength will bring our Gloom slow to 94.5%.

DURATION: We use [Primed Continuity] and Molt Efficiency. All our abilities besides Desecrate benefit from duration. At 163.5% ability duration Gloom's max drain per second is 4.59 energy, and Terrify would last 40.9 seconds, while Shadows of the Dead would have a 1.84% health drain.

RANGE: We use [Stretch] and [Augur Reach]. All of our abilities benefit from ability range.
Gloom's range is one of its most important features, since it'd be worthless if it can't affect as many enemies at possible. 28 meters is a comfy enough radius.
Terrify affects a large amount of enemies with our current ability strength, but it'd mean nothing if it doesn't have a big AoE. At 175% ability range you'll be just over the 25 meter mark, but we sadly can't add more range mods.
Desecrate lives and dies by its range, so increasing it is always optimall. 43.8 meters seems to be enough for most ocassions.
Shadows of the Dead doesn't benefit directly from range, but the links from [Shield Of Shadows] do. At 175% ability range the links have a radius of 87.5 meters, making it so you don't have to recast Shadows of the Dead to bring them close to you as often.

EFFICIENCY/ENERGY: We use [Preparation], [Primed Flow], and [Equilibrium]. With how much energy we get because of Gloom a bigger energy pool tends to be better than less energy per cast, which is why we chose [Primed Flow] over [Streamline] or [Fleeting Expertise]. Desecrate is up at all times, and it ends up creating a massive amount of energy and health orbs. Because of that we make the most out of it by putting [Equilibrium] on our [Nekros Prime], effectively filling up our energy at all times. Energy regeneration doesn't work whenever Gloom is active and slowing enemies, so our only other choice is to run things like [Netra Invocation] to make all of our other abilities consume way less energy.
As for Preparation... why use it when you can generate energy with energy orbs and health orbs non-stop? Well while it's true you won't need it when you got your run going, I've always found the hardest part of starting a high level run as [Nekros Prime] to be the very start when you can't cast lots of abilities. On top of that since we're not running [Despoil] both Gloom and Desecrate are running on the same pool. While it won't be noticeable most of the time, sometimes your energy can drain rather fast if you kill too many enemies at once. This gets rid of that issue, giving you more than enough energy to cast and use Desecrate, cast Gloom, and use Terrify a few times comfortably before casting Shadows of the Dead and getting your build going.The calculation for starting energy is:

Base Starting Energy + (5 x Mod Capacity Left) + (Total Energy x [Preparation] Multiplier).