The Psypher known as the Belderiver takes the form of a black blade tipped with a red (Cauldron-made) Psypher crystal. The lower half of the blade appears to be a normal sword, albeit a grand one. Forged by the dwarf Psyphersmith Brom, a deal was struck with Odette upon its forging: Whoever wielded the blade would be able to control the power of darkness, but their souls would belong to Odette after a set amount of time or upon their death.
The previous owners of the Belderiver are only hinted at: they seem to be uniformly psychopathic killers, slaves to the Psypher's lust for blood. This much is confirmed by the spirits Oswald meets in the Netherworld, who all seem to have perished at the blade of the Belderiver. Eventually each owner has been consumed by the blade, and their limbs have grown heavy, before Halja arrive to claim them. The claimed souls have suffered a terrible fate: Stripped of their intelligence, memories, feelings and identity, they become mindless Desecrators, moaning black wraiths seemingly permanently trapped in a warped image of the Shadow Knight form the Belderiver grants its wielder. They seem to slowly gravitate towards Winterhorn Ridge, possibly because of its nearness to the Netherworld, although Beldor was able to summon a pair of them to aid him in combat against Oswald.
Strangely, after his first death at the hands of a Halja after Melvin's ascension, Oswald does not become a Desecrator. Instead, he appears in the Land of Despair and Misery, the outskirts of the Netherworld where he is tormented by spirits slain by the Belderiver. Odette says this is because unlike the other owners, he interests her. It is also revealed the blade cannot harm Odette - it is formed from her energy. Upon his second death, he merely seems to fall into a coma under Odette's control.
If Oswald chooses the true path and fights Gallon, we see the Belderiver is able to kill the new Lord of the Netherworld- evidently Odette's blade is not connected to the Netherworld: if it was, Gallon would have been immune to its touch, saturated by the dark forces as he was. The only other path that involves the Belderiver is if he fights Onyx. He perishes and falls to the Belderiver. For a few seconds he clings to his identity and, perhaps without understanding it, says Gwendolyn's name one last time before degenerating into an empty minded Desecrator. This shows the Belderiver will automatically claim its victims as Desecrators without Odette's intervention (by this time, she is dead). The Belderiver is finally destroyed in Velvet's ritual to rejuvenate the world, or at least loses its power.
Remark - Despite Gallon's dark powers and control of the Netherworld (since he was quick to take over when Odette died), he still sensed and feared the power of Odette emanating from Oswald. Supposedly he could not match her power which would explain both how he was imprisoned for so long and how Oswald could defeat him. While Gallon and Odette both had mastery over the Netherworld there is little to suggest their powers were on par with one another.