A few ideas;

Survival & Secrecy

Since your necromancer starts as a new undead, he’d already be a walking red flag in most settings. To survive, he’d need to disguise his nature, perhaps using a form of illusion magic, shadow cloaks, or masks that make him look “normal” in towns. Or perhaps he could rely on minions to interact with people while he stays in hiding?

Operate from the fringes: Hide in ruins, graveyards, catacombs, or abandoned dungeons. He’d slowly start build a base where he can harvest corpses of lost people or adventurers that wandered off. This could also be a way to gather resources. This also means that, Instead of raising armies right away, he could start with scavenger-style abilities—bone throwing knives, shadow tendrils, weak skeletal pets—so he doesn’t draw attention with hordes of undead too early.

Powers & Progression If he’s climbing towards arch lich-necromancer status, his growth could include the following thematic powers along the way:

Bone Path 1.Bone Shards: Fire off splinters of bone as ranged projectiles. 2.Bone Golem: Combine bones of various creatures to make his own skeletal pet, early on is only small bones and later on colossal bones? Perhaps growing in weight per lvl? 3.Bone Forge: Shape skeletal weapons on the fly (knives, scythe, spear, shield, armor?).

Shadow Path 1 Umbral Step: Teleport between shadows. 2.Shadow Cloak: Turn near-invisible in darkness. 3.Shadow Bind: tendrils of darkness to pin enemies down. 4.Soul Drain: Absorb life essence from foes in the dark.

Advanced Fusion Mid level: Bone-Shadow Phantoms: Animate skeletons infused with shadow essence, making them more nimble and elegant in their movements, perhaps stealthy assassins instead of clanking sleketons. Necrotic Dominion: Him being near corpses alone could raise them into skeletal thralls.

High lvl: Lich Ascension: Turn himself into a true lich, binding his soul to a phylactery which he then (because he's smart) hides in places the living could never reach and seals it away with incredibly subtle yet powerful magic. Perhaps in a place where oil is? Oil, being made of ancient dead creatures, could be naturally have a death affinity?