Spore druids get some darky-necromancer like abilities, including the ability to raise fungal zombies. You'll want to start a class with the heavy armor proficiency if you are picturing a heavy armor "knight", gloomstalker ranger maybe choosing ranger knight for the favored enemy feature. Any race will do, but look at high elf for an extra cantrip and take chill touch for more necro-style points. I would take ranger to 5 for extra attack and the other 7 in spore druid.
Another option would be fighter/wizard, with a level break depending on how much you want to be a knight and how much you want to be a necromancer.
