This build has been tested on version 2.6.1 PTR on PC and in Season 12 on PC and console.
The build revolves around highlighting the damage boosting abilities of the Bones of Rathma 6 piece class set in tandem with the effects of the Jesseth weapon set to pile on swarming pet damage via Command Skeletons, the Command Golem, and Skeletal Mages as much as possible, burning through mobs and ganging up on elites and bosses and Rift Guardians without letting them get close to the Necromancer. The build works on pure Essence spending and recovery to maximize DPS damage, dumping it into summoning Skeletal Mages with the Singularity rune for maximum damage per hit , use of the Decay rune on the Golem to consume corpses for increased damage output, and using the Command Skeletons with the Freezing Grasp rune to crowd control enemies and proc the maximum damage bonus on Krysbin's Sentence as much as possible. The Necromancer themselves will be using Siphon Blood for damage support, healing and Essence renewal, and scooping up health orbs while casting Frailty on enemies and spamming Devour to maintain Essence and health for production of Mages and continuous activation of the Skeletons.
The 2 piece bonus for Rathma only affects Army of the Dead which isn't applicable in this build, whereas the 4 piece bonus grants 1% damage reduction for every attack by the Necromancer's pets for 15 seconds, stacking up to 50 times for 50% damage reduction. This is central to the Necromancer's defenses in this build so keeping as many pets spawned and attacking as much and as fast as possible is key. The 6 piece bonus grants a 1000% damage increase for every Skeletal Mage summoned, up to 4 mages at 4000% damage increase. All other Mages, pets, and outgoing damage benefit from this bonus so it's still optimal to keep the full 10 possible Mages summoned at once to make the most of the bonus and making sure the Mages on the field never fall below 4. The Jesseth set makes the Command Skeletons much more effective, automatically activating them to attack a new enemy once the previously activated enemy dies, saving Essence for each time and granting the Skeletons and all other pets an additional 400% damage bonus when activated. By using both class sets, this allows for a total 4400% damage increase before taking any other boosts into account.
The Skeletal Mages with Singularity rune consume all other Essence past the initial cost for 3% increased damage per point of Essence consumed. This means, say, for a total 330 Essence, 40 Essence cost to summon leaving 290 grants an 870% damage boost to that Mage. If using the Circle of Nailuj's Evol, it allows for summoning 2 Mages at once for no additional Essence cost and for an additional 4 seconds longer. More Resource Cost Reduction means less Essence spent on summoning them and more to be consumed for a higher damage bonus. The only issue is managing Essence so as not to run out and be unable to summon mages fast enough.
The Command Golem is central to the build, dealing 450% damage by itself and while the Decay rune does make it possible to buff its damage further, use of the Ice or Bone rune for different damage usage and another source of Freezing/Stunning to proc Krysbin's Sentence is always viable. Its foremost purpose though comes through the use of the Cubed Moribund Gauntlets, letting it drop a corpse once every second and paired with the corpses of killed enemies and possibly those left by Mages if using Razeth's Volition as a Shoulder option, it means there will be a constantly replenishing source for the Necromancer to Devour to restore health and Essence, moreso with the Life from Death passive and Reapers Wraps making all the dropped health orbs restore Essence as well.
The Command Skeletons work for damage, enemy aggro and crowd control with the Freezing Grasp rune, Freezing enemies when activated for 3 seconds and proccing the damage bonus from Krysbin's Sentence for an additional 300% damage bonus on top of everything else. This with the free activations from the Jesseth set and additional damage bonus as well makes it easy to keep all the buffs up as much as possible, leaving more Essence to be used for Mages and making the Skeletons effective damage dealers unto themselves.
Use of the Frailty curse is two-fold, three fold if using Dayntee's Binding for your Belt choice to add up to 50% additional damage reduction whenever at least one enemy is cursed. Frailty allows enemies to die when at or below 15% of their total health, a huge speed boost for killing mobs and other lesser enemies, but the Scent of Blood rune on it as well makes all cursed enemies take an additional 15% damage from pets, making all the outgoing pet damage that much more effective.
Devour is the main healing/Essence recovery method for the build with the Cannibalize rune, healing 3% health and 10 Essence for every corpse consumed within 60 yards. It makes for all fallen enemies a source of free resources and is able to be cast on the move and while summoning/commanding pets so it allows for easy healing and Essence replenishment at all times, especially thanks to the Golem shedding corpses every second. With Life from Death and Reapers Wraps they'll also be dropping health orbs more often than not that will greatly replenish health and Essence, and use of Siphon Blood with the Bloodsucker rune helps keep health and Essence topped off while also dealing a decent amount of damage and drawing in any health orbs within 40 yards for easy pick up.
The passives help round out all the main skills, Dark Reaping capping off 2% health and Essence when killing an enemy while using a Scythe, Grisly Tribute to heal the Necromancer 10% of their Life on Hit value for every hit dealt by a pet to dramatically boost healing, Commander of the Risen Dead reducing the cooldowns on the Golem and Skeletons for faster usage, and Life from Death giving corpses increased chance to drop health orbs when consumed. Altogether it makes the Necromancer able to keep spamming Mages and commanding the Skeletons to attack, keeping up the full 4400% damage bonus at nearly all times and keeping the Necromancer healthy.
One equipment variant would be using Razeth's Volition in the Cube or worn directly, making all Skeletal Mages leave a corpse behind when they expire or die. This would allow them to help earn back their initial Essence cost and help mitigate the expenditure and recovery gap even more, but it would require using the Ring of Royal Grandeur to maintain the full Rathma set bonus, eliminating most other Ring choices. If using in the Cube, it would require wearing the Moribund Gauntlets be worn directly. In the instance of using the Ring of Royal Grandeur in the Cube, I'd recommend wearing a maxed Circle of Nailuj's Evol in place of Convection of Elements, and this would allow an alternative rune for the Golem like Ice or Bone for another means of proccing Krysbin's Sentence with their Freeze or Stun effects.
Another variation that would have the Moribund Gauntlets worn would involve Cubing the Mask of Scarlet Death and swapping the Devour with Cannibalize skill with Revive with Purgatory skill, allowing an additional empowered minion to be summoned either from monster corpses or the free corpses shed by the Golem. This decreases the build's healing/Essence restoration at the offset of another high power offensive minion, the Mask of Scarlet Death combining all used corpses into a single monster with an additional 150% damage increase per corpse and the raised minion would benefit from the Rathma's 6 piece bonus of up to 4000% damage and the Jesseth set bonus of 400% damage. The Purgatory rune would allow the used corpses to be recycled after the minion died or expired, and would thus also grant a near constant set of health globes from them for more health/Essence restoration on the offset, especially as multiple minions from different groupings of corpses could still be maintained simultaneously instead of just one at a time.
While this is a solo build in most terms, the sheer output of health orbs from Life from Death and all the free corpses dropped by the Golem can keep a party fully healed in group play, and the Frailty curse affects all forms of incoming damage to enemies, not just from the Necromancer or their pets so the build is indeed viable for group play.