According to If you True Polymorph a zombie you create via Finger of Death, does it remain under your command? after a necromancer polymorphs an undead they control, it is still under their control. Assuming yes, I raise you an even more overpowered combo than a bunch of pixies running around:
A necromancer gets a permanent zombie via Finger of Death.
The zombie gets polymorphed into an object via True Polymorph.
After taking a long rest, the necromancer uses True Polymorph again, this time going for the "Object into Creature" option, turning their permanent zombie into any creature with a CR of 9 or less.
Is this possible RAW?