Your studies of ancient grimoires have given you inspiration to the untapped potential of the dead. Why waste your magic on mindless minions when the dead could be used for a much greater purpose. You. You see that the dead have the means for providing, healing, durability, strength, and most of all power.

This subclass was designed to be used with the custom necromancer 5e class available at "D&D Wiki" (I can't link because DND Beyond wont let me).

Undead Surge

Beginning when you select this school at 2nd level, While you have temporary hit points you may use this ability to gain an additional bonus action this turn. Then for each bonus action available at the end of this turn add 1 to your AC until your next turn or until after an attack roll has been made against you. You may only use this feature once per turn and only a number of times per long rest equal to your proficiency bonus.

Deathly Reconstitution

At 2nd level, you gain the ability to utilise your fallen enemies to mend yourself. You may repurpose the flesh of a nearby corpse to repair damage to your body. When you roll hit die during a short rest, excess health may be applied as temporary hit points (the temporary hit points are equal to the sum of excess healing from all hit die even if hit die are rolled separately).

One Man Horde

At 6th level, You may undergo a horrific metamorphosis and assimilate your undead horde into your body gaining you strength and durability.

Whenever you cast animate dead you may fuse with the creature/s. Choose any number of physical parts from and up to the available limbs/appendages/etc. Your character will adopt these parts on their body wherever you choose. If a specific part is required for that creatures attack to be possible but is not chosen then that attack cannot be made. You cannot replace an existing body part unless that part is already missing. Your character gains the following benefits:

• If the creature raised is a larger size then you then your size becomes that creatures size.
• You gain temporary hit points equal to half the total health of the assimilated creatures.
• While you have temporary hit points you may use a bonus action to make an attack with each undead creature assimilated.
  • NOTE: You still use the monsters hit and damage characteristics when making an attack.
• You have advantage on intimidation checks but disadvantage on persuasion checks.
• Undead infused with you are permanently under your control.
• If you take damage from a single hit that exceeds half the total health of the assimilated creatures then the assimilated limbs are destroyed and you lose all effects of One Man Horde for a minute after which you may until you re-summon and assimilate new undead.
• While within 5 ft of an enemy You may spend temporary hit points equal to three times a spells level to make a ranged spell attack at that enemy without disadvantage (cantrips cost 2 temp hp).
• When attacking as a reaction you may substitute your attack for one attack an assimilated creature could make.
• You may revert this assimilation as a bonus action. This process destroys all undead assimilated.

Adapt Physique

Beginning at 10th level, you may pick a characteristic from among the characteristics of all absorbed undead (other than immunities). After a minute of reconstructing your body That characteristic applies to your character until you change it or your One Man Horde is undone (You must use this feature again to regain characteristics). You may have up to two different characteristics applied at one time. You may use this feature once per long rest.

You may use this feature twice per long rest one you reach level 14

Become Monstrous

Starting at 14th level, you can boost yourself with magic to enhance your assimilated undead. As an action, your assimilated undead swell in size and for one minute you gain the following benefits.

• You may choose another characteristic from the characteristics listed among the assimilated undead and use that characteristic for the duration of Become Monstrous.
• Your melee attacks are considered magical for the purposes of immunities and resistances.
• If your size is smaller than large then you become large.
• Your strength modifier is equal to your intelligence modifier.
• You have resistance to bludgeoning, slashing, and piercing damage from non-magical attacks while you have temporary hit points.