There's no Necromancer faction.
To be precise, there is no "Necromancer guild."
He talks about that there are four "main factions" and then there are smaller groups, and the Necromancers could be one of these smaller groups - hopefully at least they are not automatically hostile, mindless enemies, but rather some sort of possibility of dialogue options and maybe even some quests involving this group.
"Necromancers - yes, there are Necromancers in the game. The four main factions - four main factions?
Yeah, four "main" is the right way to phrase that because there are some smaller groups that you can see and come across, but the four main factions that you actually join and have stories and stuff related to them are the Warrior's Guild, the Mage's Guild, the Thieves' Guild and the beloved Dark Brotherhood. There is no Necromancer's Guild. That shouldn't be a surprise to anybody. But they're in the game, they do have spells and they are spells that you can find and use."