In this section we discuss magic in two parts. Buffing and PVP magic.
Buffing:
Necromancers have no real offensive buffs, but they do have strong defensive buffs. A brief breakdown of ones that are especially helpful in PVP.
Kura-Silma (KS) has an aspect of it that gives your Necro a perception bonus, which is always helpful when fighting other stealthy characters. It takes two spells to cap duration at 29 minutes, but due to its length, it's a pretty good choice for the first buff spell you cast since it will still probably outlast all of your other buffs. This spell works nicely or "stacks" with other Perception boosting spells, like CV.
Calcified Hide (CH) is currently a damage suppressor. It's great against characters that will otherwise just get hammer;throw hammer smoke you. What's even better is that it works in conjunction with Manifest Force, a learnable spell that does pretty much the same thing. You can have them both up and they'll both eat up incoming physical damage.
Philosopher's Preservation (PHP) is an evasion bonus as well as a Reflex Boost. It's a terrific spell. This and the CH/MAF combo really increase your survivability. As both CH and PHP have about the same max duration, it's normally just cast them back to back and get out of the fight when they come off.
Obfuscation (OBF) is a hiding and stalking bonus. It's not as powerful as the Moon Mage Shadows spell in terms of a hiding bonus as it does two skills, but it's useful. This does stack with Shadows if you have a MM buddy or some Azurite runes and don't mind possibly exploding your arms.
Eyes of the Blind (EOTB) is Necromancer inviso and for my money it's the best Invisibility in the game. The Pros are myriad. Good duration length without limitations on where it's effective or requiring held mana, it's an essential for any time you're in town, combat or not. You cannot be purged while invisible. It also will automatically pulse you back into invisibility if you come out for some reason, either pointed, casting, attacking, etc. Should you be disabled, i.e. slept by an Empath, this alone can sometimes keep you alive, since they'll have a difficult time getting their avenger to melee on you. The ability to move in and out of a room without detection is key for an ambush. Stalking is a nice skill, but in terms of just moving around your battlefield, this is a Must Use.
Worm's Mist (WORM) is a tremendously powerful magic barrier spell. It nerfs outgoing and ingoing magic, but you do have the option of casting the spell NEAR or FAR. NEAR puts the spell at melee range, meaning all your spells will be killed or minimized. FAR will let you self cast without interference, but anyone who gets to melee range on you will be able to cast on you freely as well. Due to its short duration (10 minutes max), it's probably the last spell you want to cast before entering combat. Additionally, since WORM is a double-edged sword, it is not for blanket application regardless of who you are fighting. If you know you can shoot a dude, but he might cast back at you and disable you before you finish, it's really a nice choice. If you need to be able to use your own magic, you probably don't want to nerf yourself. The situations for Worm really could be a post in and of itself, but as a basic tenent for when and when not to cast it, "Know Thy Enemy."