This build was designed for 2.6.1 and tested in the 2.6.4 PTR.
The build revolves around the Cubed Nayr's Black Death scythe, changed for 2.6.1 to boost damage up to 100% for every different Poison element skill used for 15 seconds, refreshable with every different skill rotated through. Thus the ideal is to balance as many Poison skills as effectively as possible. The build is partially a generator build, as it focuses on Grim Scythe with the Cursed Scythe rune and the 2.6.1 updated Leger's Disdain for much of its direct damage and using the other skills as Essence dumps and reinforcement damage. Leger's Disdain now boosts damage of Grim Scythe up to 80% for every point of Essence it restores, 12 Essence from a single enemy but generating more when striking multiple enemies on a single swing, and thus more damage, but only when not at maximum Essence, so continuously spending Essence is key.
Bone Armor, given that this is the Inarius set, is the most potent boost for damage, as with Dislocation allowing for an on-demand Stun effect to proc Krysbin's full damage bonus, the bone storm is also Poison element, with a 1000% damage proc on first use, then on every tick as well as enemies being struck by the bone storm taking on an additional 3750% damage from the Necromancer from all skills. Keeping enemies within the 15 yard radius of the bone storm ensures all other outgoing damage receives the boost along with the bone storm's own damage proccing.
Skeletal Mage is the Essence spender here, and while the Singularity rune is indeed an option as it allows the Necromancer to dump all their Essence in one shot, the skill rune makes the summoned Mage register as Physical element damage, so it wouldn't benefit at all from the total 600% boost to Poison damage. Instead, the Contamination rune is key, and it works with the slower moving, mob-mowing approach of the build very well. The summoned mage radiates a 20 yard radius Poison aura at 100% of weapon damage at base for the duration of its summon in the location it was summoned, and the summoned mages' damage auras all stack if multiple mages are summoned in a single spot. This skill you will be spamming non-stop, either manually if you are on console or using the num-lock trick for PC if you aren't as fast and consistent on using it. As long as you have Essence to spend, you spend it on this. It piles Poison damage right in the perfect radius around the Necromancer to stack with all the other outgoing damage, and even moving from place to place while summoning keeps spreading the effects of Efficacious Toxin and Bane of the Trapped around to enemies.
With Razeth's Volition in the Cube, this also means that you're stacking free corpses right around the Necromancer as the Mages expire and enemies die from them and all the other outgoing attacks, right in perfect range to use for Corpse Explosion with Close Quarters, benefiting from the 600% Rayr's boost and the 3750% bone storm boost being within the 15 yard radius. Unlike some other Poison focused builds, this is not meant to be the main source of outgoing damage, but to pile on as much damage as possible as an AoE effect to all the enemies grouped around the Necromancer.
Bone Spirit with the Panic Attack rune is a high damage freebie thanks to all the corpses being consumed shredding through its cooldown, allowing for Fearing enemies for the Krysbin's full damage bonus at will and also benefiting from all the boosted Poison damage and the bone storm bonus at closer ranges. Another option to player preference involves equipping a Follower with the Pandemonium Loop, as the ability will still affect deaths of enemies Feared by the Bone Spirit even if the Necromancer isn't wearing the ring directly. This option is preferable over the Necromancer using the ring directly as it would mean giving up one of the rings needed to balance the build properly.
Army of the Dead was originally used with this build, but with some of the changes to the Rift ranges, a speedier cycle of all the buffed Poison damage is definitely preferred and has been swapped with Death Nova with the Blight rune for a bit of additional damage radius and effect control. Paired with the Skeletal Mages, it should offset the Essence gain-to-expenditure rate to keep getting as much viable damage out of Grim Scythe as possible with Leger's Disdain.
The passives have been retooled for 2.6.4, with Life from Death now taking priority for survival and Rigor Mortis added for additional crowd control function. Dark Reaping and Draw Life are still central to the build, and if using a Hellfire Amulet, Gristly Tribute would be a good choice for all the Skeletal Mage summoning.
As this build warrants slightly slower play and hacking through mobs as much as elites, this is definitely a solo oriented build and should not be attempted for group play involving speed farming or major GR pushing.