Necromancer are offensive spellcasters specializing into dark magic. They can use all weapons and wear robes.
Necromancers have a broad palette of curses, especially Curse Discord to make one enemy attack another, Curse Fear to make them flee, Curse Poison for damage over time, and the powerful Anchor spell which paralyzes the victim, as well as Forget to make victims unaware of previous attacks, plus quite a number of Debuffs. Against other spellcasters, they have the special spell Silence.
As nukers, they get the spells Vampiric Claw, which does much damage and heals the caster from it, but has a high mana cost, and Death Spike, which costs very little mana, but consumes a cursed bone which one can't buy in the shop and is only dropped sometimes by certain undead opponents, and later Curse Death Link, which has ok damage and cost, but most importantly gets stronger the lower the hp of the caster drops. They can also use Corpse Burst to let fallen victims explode and damage bystanders. Their other mass spells are Mass Slow and Poisonous Cloud.
They can also do some summoning (Summon Corrupted Man for 90% xp loss, Summon Reanimated Man for 30% xp loss), but they lack all the spells to support their summons any further, like healing and buffing, so they will have to rely on their lowlevel spells alone to strengthen the summon, and directly help their summon with their debuff magic. They can however use Transfer Pain to move damage made on them into their summon.
 PVP Scenarios and Behavior
The Necromancer class has received a lot of notoriety for being one of the strongest classes when it comes to PVP. Many Lineage II players have been clamoring for the class to weakened. However, changes have yet to be seen.
In PVP the Necromancer will always first prioritize disabling his or her opponent with either a slow, or sleep spell. Sleep being the preferred spell because most of the Necro's essential anti - player debuffs will not cause the target to wake up. Once the Necromancer is in a presumably good distance. (Out of range if possible.) The aforementioned debuffs will be put into place. The first priority debuff is Curse: Gloom which greatly lowers the opponent's magical defense. And more importantly, hardly ever misses. (More likely, given a small level difference between opponents - never misses at all.) The next is Silence to prevent the target from casting spells or using skills. And then for against archers Curse: Weakness, against melee fighters Anchor. And after that it's just an elementary succession of damage spells like death spike or vampiric claw.
In high speed PVP battles, normally against other nukers, (especially the fast casting spellsinger) some Necromancers prefer just using Curse: Gloom and bombarding the enemy for a quick victory. However, Necromancers rarely do this as they will not risk their spells being canceled.
 Previous Comment
The necromancer is a very diverse class. Not only are they capable of high level sumon magic like their Warlock counterparts, but have the hardest hitting spell available for human caster classes. Many consider the class a middle ground between a Spellhowler and the previously stated Warlock for their use of dark magic and summon skills.
As well as the types of magic already included, the necro has access to more curses of any other class, including the ability to push hate between mobs, paralyzing enemy PC's and a variaty of DOTs (Damage over Time). Not a specialized class but rather one with options.