A bit late but relevant to my interests.
I've been hankering for a game that allows you to act as a necromancer or commander of undead, whether RTS, TBS or even something along the lines of Diablo 2.
Bonus points for being able to use your enemy's deaths to expand your ever growing army, to the point of becoming an undead singularity of sorts.
Dungeon Crawl Stone Soup. If you worship either Kikubaaqudgha (suitable for caster necro archetype) or Yredelemnul (more of an evil-priest vibe, basically a non-caster's way to get undead servants) you could do something akin to a necromancer. Both allow for some undead commanding.
Warcraft 3 Undead are pretty good for it as has been said.
Sacrifice allows you to command a bunch of monsters no matter which school you go for, though necromancy monsters are themed appropriately. You do get a spell that lets you revive dead
friendlies, appropriately titled Animate Dead. Overall it looks the part but mostly feels like a general wizard/summoning thing with an appropriate theme rather than through and through necromancy sim.
Dominions 4 has some pretty cool undead stuff if you don't mind the visuals. Do give it a look. There's a good number of death-magic nations and you can do some interesting things with those. Same devs have made Conquest of Elysium 4 which should also have a necromancer class to play with. I've only played CoE3, which was neat but flawed. Apparently CoE4 is a bit more neat and a little less flawed, though still very much both. Worth a gander.
I'm not entirely sold on Heroes of Might & Magic undead myself, but they do look the part and games are fun nevertheless.
For your singularity point, I think Dominions and HoMM are most suitable. HoMM necromancers can convert enemy dead into yet more skeletons at a... 3:1 ratio? I forget. And Dominion has a few nations that 'radiate' death and spawn more undead.