Our Necromancer is a reworked class from the PHB. With a focus on prolonged control and more reasonable undead characteristics.

Necromancer Savant

Beginning when you select this school at 2nd level, the gold and time you must spend to copy a necromancy spell into your spellbook is halved.

Grim Harvest

Beginning at 2nd level, you learn the ability to reap the life force from dangerous foes. After damaging an enemy with a spell, restore 1/8th or 12.5% of the damage dealt as hit points. If already at full hit points gain up to 2 times your Necromancer level as temporary hit points. 

Temporary hit points disappear after 1 minute without proccing Grim Harvest or losing them in combat.

Undead constructs (or other summoned creatures) that you control do not grant Grim Harvest. 

Grim Harvest may be used multiple times during a spell cast. E.g. An area of effect spell affecting more than 1 enemy. Add all damage dealt to enemies and apply reduction.

Grim Harvest does not apply to enemy undead as they do not have life to reap.

Undead Thralls

Starting at 6th level, you may add the Animate Dead spell to your spellbook if it is not there already and the ability to cast it as a ritual once per day.

When you cast animate dead, you can target one additional corpse or pile of bones, creating another zombie or skeleton, as appropriate.

Whenever you create an undead using a necromancy spell, it has additional benefits: The creature's hit point maximum is increased by an amount equal to your wizard level and adds your proficiency bonus to its attack rolls. 

Animate Dead (Necromancer Ritual) - when you raise an undead construct they remain under your control forever. After 24hrs the construct's hit points decay by 2d4, every 24 hours until the construct's hit points decay to 0. After which the construct will turn to inert dust.

Construct current hit points are not able to regenerate by any means (with the exemption of Necrotic Wake).

Soul Siphon

Beginning at 10th level, you can add an additional damage die when damaging enemies under 25% or 1 quarter of their hit point maximum. The additional damage type becomes necrotic.

At 15th level, you may add an additional damage die. E.G. Casting Fireball at 3rd level deals 8d6 Fire damage and 2d6 Necrotic Damage to enemies under 25% of their maximum hit points.

Necrotic Wake

Starting at 14th level, you may release necrotic energy in a wave. All enemies (chosen by you) in a radius of 60ft, centered on you must succeed on a constitution saving throw against your spellcasting modifier or take 12d6 Necrotic damage or half on a successful save. 

Any undead constructs under your control within the radius receive hit points equal to half the amount rolled.

Necrotic Wake is able to trigger Soul Siphon on multiple targets and Grim Harvest healing. The healing to Undead Constructs from Necrotic Wake is not affected by the Undead Thralls limitation.

Once you use this feature, you can't use it again until you finish a long rest.