The name 'Dark Order' refers to any organization of evil mages attempting dominance of one or more worlds.
The Slayer Dragons have faced and destroyed numerous evil magicians or manifestations of evil magic, over the Ages of the Alliance; most of these were overcome, and the only serious threat was that of Samuel Fate. Most 'Dark Orders' are white-skinned, black-robed sorcerers or warlocks, obsessed with gaining power and often superstitious in the extreme. Where they cannot practice openly (as a result of industrialism preventing widespread magic), they augment social corruption. Their use of magic is consistent with records (discovered in the Sixth Age) of the Iname.
Dark Orders form a loose coalition under a single leader, called the 'Dark Lord' (not to be confused with the immortals of that name); itself often male, but not always so. Apostates are killed instantly. Death of a Dark Lord usually means death to most of its followers; therefore a successive Dark Order, on the same world, fails to learn from its predecessor's mistakes. Ranks within a Dark Order are ill-defined; but the more powerful a member is in magic, the higher its status. Envy is commonplace; but struggles for power are averted by the Dark Lord, who remains the sole internal authority.
Use of Magic
As a rule, single members are magically weak, but the group is strong. Their magic consists largely of causing, exploiting, and maintaining fear in a victim, whereby the victim is made subject to their wills. As a result, most Dark Order believe themselves inheritors of the Iname's power. Ranking members are awed and frightened by the Dark Lord's abilities (which slightly exceed their own). Few Dark Orders practice magic except before a victim; when they do, it resembles Tower Magic.
Connection to Necromancy
At least one Dark Order, and possibly more, have contained remnants of the Grey Temple, and experiment therefore in genetic manipulation (usually meant to produce monsters). The last known iteration thereof was destroyed by Marcus Ransom in the Fourth Age. Unlike true Necromancers, Dark Order seldom paint their faces (except in cultures requiring this), and typically dress in very-plain clothes.