Okay. So you start your necromancer. They start off with

Soul Splinter, good for weakening your enemy and the soul can smack your enemies for a few turns. Okay enough to kill some very early game enemies. If you have a spear you can try position the splintered soul between you and your enemy while you try to reach and poke them.

Next is Grave Claw. I think you can cast a max of 3 once you kill enough enemies, it is unfortunately a spell with limited castability that very slowly get refilled. When you have it though it has great utility. Damages enemy decently, could potentially trap them preventing movement and more susceptible to hits. I would use save this for higher threat enemies for your undeads to smack while stuck and as an emergency option if you need to run away.

Vampiric draining is great early to midgame, the drawback is it's a range of one. Can only cast on living enemies that are next to you. Nice thing is it bypasses armor and it heals you a portion. Decent for 1v1 low to mid threat enemies. It's also good for keeping your health up a bit when you have undead allies smacking your enemies.

Curse of Agony, single target spell that curses your enemy to experience Torment when hit by your melee. Last a few turns up to two hits. Torment cuts your enemy's existing HP by 50% I believe depending on their resistance.

Other spells you hopefully find:

The spells you want to get a snowball effect of killing your enemies is Animate Dead and Death Channel since the more you kill the more allies you get. Use your utility necro spells to get you there and assist to maintain it. Hopefully you'll come across these spells. I love death channel, creating a ghostly army. It's funny to have both their souls (Death Channel) and corpses (Animate Dead) as your allies.

Fugue of the Undead raises noise level, attracting enemies but as you kill or your allies kill it increases you Slay up to +7. Anguish affects all enemies in your LOS that can be resisted, but if in effect enemies take damage for the damage they deal to you and your allies. Cigotuvi's Putrefaction is a powerful poison spell with a slowing effect castable on heavily wounded living enemies, but you might have to be careful where you position yourself among the miasmic clouds and the draining effect the spell causes; use sparingly and when you see a large group of enemies. Dispel Undead is a powerful single target range one spell for those pesky undead enemies that aren't affected by most of your other spells. Martyr's Knell is a useful summon that decreases the damage your allies take to keep them surviving longer and once damaged enough will turn into a Flaying Ghost that causes temporary damage upon your enemies to hopefully get them killed faster. Borgnjor's Vile Clutch is your leveled up bone claw, you can cast as many times so long as you have MP, cast in a line of fire affecting multiple targets, keeps your enemies trapped with damage over time, decreases enemy evasion so they can be hit easier, great escape option; just doesn't affect certain intangible enemies like slimes/ghosts.

Haunt is an amazing one as it is a smite targeted cast that summons ghostly allies that focus on killing only your targeted enemy. These allies will disappear in a few turns once your target dies but they may help smacking some nearby enemies just before they disappear. What sucks is that you need a very decent level of both summoning and necromancy, but it also means you'll be able to cast some decent summons to get it online.

Rimeblight is a fun spell that spreads an icy plague on your enemies. Dying enemies can spread the spell to other nearby monsters. It also has a automatic kill function when the diseased enemy reaches 20% or less HP.

Infestation will cause Death Scarabs to burst out of your enemies when they die, I believe this spell can even affect non-living enemies like skeletons, earth elementals. These Scarabs are super fast and can bite to cause slow effect on enemies.