Transform into a lich at the press of a button.
The necromancer is the master of death magic, using their frightful undead pets and powerful disease-based damage-over-time spells to kill anything in their path. They're one of the most powerful solo classes, but their damage is significant enough that they are desirable in groups. They unfortunately lack group utility, but they are otherwise a very strong class.
Their fears are the best in the game, making them particularly capable in outdoor situations. They have many special abilities related to undead, and are able to fear, charm, slow and mesmerize them. This makes them a particularly useful group class for dungeons chock full of the restless dead.
Necromancers can also heal their pet and lifetap enemies to regain their own health. They're unusually survivable for a caster class, with high hit point totals and the aforementioned capability of stealing life from their targets. They can also transform into an undead form themselves, slowly draining their hit points to go into mana. They then use lifetaps while in this form to keep them topped off and deadly. Necromancers can even heal others in their group, apply a powerful poison resist buff, resurrect and feign death. They're quite self sufficient, and it's a good thing - all necromancers receive a heavy faction penalty, making it difficult for them to travel around to the various cities around Norrath. They can make this up by doing a lot of faction grinding, however.
Their high damage-per-second and ability to manage threat makes them high in demand in a raid setting. Their damage-over-time spells all stack, making for a steady but substantial tick of damage. Their various forms and lifetaps ensure that they take little resources from the rest of the raid, and keep themselves alive at the same time. Their most important statistic is intelligence, followed by stamina for additional hitpoints to convert to mana. While they are a challenging class to play, they're also quite rewarding, and second in solo ability only to the druid.
Necromancers also have spells that return substantial mana to their group, making them an excellent choice for a caster damage group in raids. Mostly, they are desirable because they have almost no issues with threat, so that they're a class that is as self sufficient in group play as they are alone.
They're an excellent class choice for people looking for a versatile, complex and useful damaging caster class with a strong pet.