I have a strong suspicion that Desukan is an evo of Spiritomb. I mean, look at Spiritomb's dex entry:
It was formed by uniting 108 spirits. It has been bound to the odd keystone to keep it from doing any mischief.
Desukan has ghostly arms coming out of it, and to me that means that there are multiple spirits within it, similar to Spiritomb. Not only that, but they seem to be locked away within the sarcophagus, because they are evil, mischievous spirits. They are trying to escape from their prison. So I'm thinking Spiritomb evolves into it, maybe by a new method, like you find a sarcophagus in those ancient ruins at the northern tip of Isshu and bring Spiritomb there and it enters the sarcophagus, evolving into Desukan.
Oh, and just a random thing I'd like to say: It's really annoying that people keep saying pokemon are departing too much from animals, or they "don't look like pokemon". They are called "Pocket Monsters", not "Pocket Animals". And what about Grimer, Magnemite, Voltorb, etc? Anyways...