A mildly ornate box made of black oak, able to be carried under a single arm with effort. Gold and silver metalwork covers most of the box with eye-twisting patterns that cause headaches for any who stare for too long. The lid is locked shut, requiring either the key be used or that the defensive magics laid into it be dispelled before the lock can be picked. Someone went to a decent bit of trouble to make sure the contents weren't easily accessed.
Necromancers are notorious powermongers. Rare is the person who wields the destructive, subversive power of undeath that hasn't yet been corrupted and seduced by it. Kiras'thel was no exception, and his thirst for power was just as great as any one could find. But as with all those who seek power, he was opposed by various rivals. One whom he clashed with many times was known as Warmaker, an appellation earned by his habit of raising large armies to utterly ravage the countryside. Eventually their final confrontation came, and Warmaker was defeated, for Kiras'thel was the stronger of the two. However, the beaten necromancer was not slaughtered as most rival practicioners are, for Kiras'thel had a better use for his rival than being worm-food.
Warmakers heart was carved from his chest, and his brain from his skull. Mind and soul were bound to a specially-prepared box, where all that knowledge of creating and controlling undead armies could be called upon at-will by Kiras'thel. Kiras'thel was eventually defeated, but the Reliquary was overlooked and eventually lost.
The Reliquary of the Rotting Legion has a number of different capabilities that can be employed by its possessor.
It's primary effect is to enhance and reinforce nearby undead; after all, it's always easier to prevent damage to minions in the first place than to raise new ones later. Shade, bone and rotten flesh alike become noticably faster, their durability higher, and most disturbing of all, their mindless eyes become brighter, at least slightly more aware of their surroundings. Covering yourself in rotting zombie flesh won't let you sneak past these abominations.
Additionally, the bearer of the Relinquary is able to raise a powerful type of undead that Warmaker found useful for breaking fortifications and other secured locations: the Ossilyth. A tank of a creation, the Ossilyth is heavily armored and capable of simply barrelling through the wall of a building.
Most insidious, however, is the seeping, festering aura of death that emanates from the Reliquary. Those undead that fall nearby will rise again within a short time, once more ready to serve their dark master.
Unknown to Kiras'thel, however, the magics that bound Warmaker to the box were flawed, allowing his bound essence to exert influence over the undead enhanced and raised by the box. The spirit avoided taking advantage of this while Kiras'thel lived due to fear the flaws would be repaired. Now, however, Warmaker will occasionally struggle against the current possessor of the box, attempting to take control over the undead and wreak havoc on the countryside.