School of Sundering

The School of Sundering is similar to that of general necromancy, but through either mad experimentation or ignorance, the caster horribly botched the animation of their first undead. A part of the necromancer's very soul has become permanently fused to the undead, binding it to and allowing it to grow stronger with the caster, but rendering them unable to animate or even control a single other creature. 

Necromancy 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.

Manikin

The creation of even the most basic undead was still beyond you, yet you still tried, and have paid the price.  The Manikin is the resulting aberration: a simple skeleton with a fragment of your soul bound to it, granting it the potential to learn and bind new materials to itself. Although your control over it is by nature unbreakable, always remember: it is still an independent undead creature, and holds an innate malice towards all living things. https://www.dndbeyond.com/monsters/4548884-manikin

In combat, the Manikin shares your initiative count, but takes its turn immediately after yours. It can move and use its reaction on its own, and you can take a bonus action on your turn to command it to take a specific action. That action can be one in its stat block or some other action. If you do not, the Manikin instead uses its action to attack the nearest living target, including friendly ones.  If you are incapacitated, the Manikin will instead attack the closest hostile creature to you and act in an attempt to protect you.

If you take a short or long rest, (abilities with an equivalent effect don't count) you take some of that time to perform maintenance on the Manikin, healing it for a flat half of it's maximum hp or all of it's maximum hp respectively.

The Manikin can never be truly destroyed as long as you still live, and when reaching 0 hitpoints, is simply considered unconscious until healed by a short or long rest. During a long rest, if it's body was completely obliterated or left behind during the day for any reason, it will magically reconstitute at the cost of half your maximum hp and your two highest spell slots. You regain those hitpoints after 24 hours.

Reformation & Ossification

After countless hours of thought and experimentation, the ideal form of your Manikin has been made clear, and, armed with enough magical prowess to practice real necromancy, it's time to take the first steps towards that design. Using monster parts, artifacts, and gear you've collected, make major (decided by the dm) changes to the Manikin, possibly adding completely new abilities, changing it's entire body shape, and many other possibilities. This will permanently set the course of its development.

Inured to Undeath

Beginning at 10th level, you have resistance to necrotic damage, and your hit point maximum can't be reduced. You have spent so much time tinkering with your Manikin and the forces that animate it that you have become inured to some of their worst effects.

Supreme Undead

Having undergone many qualitive changes in body and mind, your Manikin has long since ascended past being a mere skeleton, and has instead become a higher form of undead. All undead recognize this status, and will refuse to attack the Manikin unless magically compelled to. As you are bound in soul, this authority naturally extends to the owner as well.

If a hostile undead has an Intelligence of 8 or higher, it can make a charisma saving throw against the owner's spell save DC to ignore that authority. If it has an Intelligence of 12 or higher, that saving throw rolls with advantage.