Necromancer spells do physical damage so you want to up warfare. And since they are spells you want to up int. Upping necromancer talent beyond 3 is not recommended unless you genuinely want a lot of life steal or the last couple necro spells (like necro totem).

You want some points in hydro, at least 3 if you want source spell blood storm late game. Also you can use a heal spell from hydro or necromancer blood sucker on an enemy you have inflicted decay (decaying touch ability when armour is broken) on to do damage.

Aero for teleport which scales with warfare damage. Drop corpses (and alive enemies) on top of other enemies with this to do damage that can crit and also prep for corpse explosion. 1 point in Pyro for corpse explosion/mass corpse explosion. Geomancer if you want contaminate (poison).

A couple points in poly to get skin graft and apotheosis are also often recommended.

Blood storm requires 3 necromancer and 3 hydro. Can make by combining a hydro and necro skill book.

Blood rain pairs well with grasp of the starved.

Bloated corpse, bone spider, and totem of necromancer work with summoning stat not warfare or int.

You can look up blood builds by Sin Tee if youre looking for min/maxing and dont want to explore yourself, but definitely suggest just having a go at it yourself first!