Death Magic

Death Magic is a Necromancer secondary attribute meaning that any character with a Necromancer for their primary or secondary class can put points into this attribute.

Guild Wars Description
"No inherent effect. Many Necromancer skills, especially spells which raise undead servants, manipulate corpses, or deal cold damage, become more effective with higher Death Magic."

General Description
The Guild Wars description hits the nail on the head for once in its description for this attribute. Death Magic is useful in three ways: undead minions, manipulating corpses, and dealing direct damage.

Undead minions - that's what Death Magic is all about. There are currently three skills that raise minions from a corpse: Animate Bone Fiend, Animate Bone Horror, and Animate Bone Minions. All are different in how they function, but basically they all raise up another ally for you. Also, Death Magic possesses many skills to help your minions, or keep you alive using them. These include Blood of the Master and Verata's Sacrifice to heal all your minions, and Infuse Condition and Taste of Death to protect yourself at the cost of your minions.

Other then raising minions from corpses, Death Magic skills also can manipulate corpses in other ways. Using Necrotic Traversal or Putrid Explosion is a great way to deal damage to hordes of foes at once. Consume Corpse and Soul Feast both utilize a corpse to heal you. Finally, putting up a Well of Suffering or a Well of the Profane using a corpse is a great way to put the hurt on your opponents.

Finally, Death Magic can deal direct damage in a variety of ways. Deathly Chill and Deathly Swarm both deal some cold damage to your foes, while Vile Touch deals unblockable shadow damage. Death Nova turns your allies (or yourself) into Bob-Ombs, and Rotting Flesh gives your target disease for quite a while.