When he died, the Archbishop wished to be cremated upon a pyre both because such was the Jovian Way, and to prevent anybody from taking revenge upon his inert remains, but two of the undertakers who worked on his body were secret necromancers. In secret, they beheaded the body and replaced the head with a fake one made of wax and based around a real human skull stolen from a graveyard. They worked on it for months, first summoning his soul and trapping it in the skull to make it a thing of High Magic, then melting down stolen Funeral Gold and Grave Silver to plate it with, until they had an item that could summon all the dead within earshot.
Not just anybody, of course, could just summon their own Undead army and command it properly of course. To do so, the necromancer would need to be skilled at his or her craft, wear enchanted Boots of the Moon and wear an Enchanting Glove. Only then would the dead be firmly under the summoner's total control. If the average person was just to walk into a graveyard and use it, then he or she would be the very first victim of the Undead that were so stupidly summoned. But if the summoner was to lock themselves in an Undead-proof building or draw a proper pentacle that kept the Undead out, then the Undead would cause some proper chaos and slaughter every living being in sight, and those they killed would themselves raise as Undead to cause yet more chaos.
The Bone Idol was first used by the skilled necromancer Boudicca, who walked into the main graveyard of Grand City wearing Boots of the Moon and a silver Glove of Undead Entrancing. More then ten thousand Undead were raised as zombies and skeletons. Those that were bones only were brittle-and even the somewhat fresher Undead were not hugely strong, but there were a lot of them, and those they slew would rise as Undead themselves. The townsfolk were slaughtered, the hastily summoned Grand City Militia were defeated as well and the armoury was looted. Boudicca walked from town to town rising the Undead and razing the towns, taking whatever weapons she could find to arm her Undead horde with, and it took more then two years, and eight Queen's Guard Legions to defeat her.
Rather then destroying the Bone Idol, as perhaps they should have done, the government decided to keep it for themselves and study it, and a greedy government minister who wanted to rule stole it and used it to summon the Undead. But he lacked the Boots of the Moon, the Entrancing Glove, or the proper skills of necromancy, and he became the first victim of the Undead that he summoned. It took two days to cauterize the Undead contagion that he released. This time the Bone Idol was hidden in a small out of the way temple of Mathom, the God of Delays in the hope that His influence would keep it hidden.
In the hands of a skilled necromancer with Boots of the Moon and an Entrancing Glove, the skull when blown through the top of it produces a loud and horrible scream of anguish, and summons every dead body within earshot, able to raise all the dead in even a large cemetery within minutes, and the dead can be controlled. The skull when held greatly reduces the holder's fear.
In the hands of anyone else, the skull does the same thing but the dead just randomly attack everyone in sight and then wander randomly all over the place causing chaos and panic.
The PCs are very unlikely to have the skills and items needed to create their own Undead Army, and will most likely be slaughtered if they try by the Undead they summoned. But if they can flee in time or put themselves in an Undead-proof place, then they will be safe whilst an entire graveyard of Undead erupts from the ground and whales on their foes, stopping any pursuit by law enforcement or an angry mob in it's tracks, and making everyone in the area either flee or be tangled up dealing with hundreds of Chaotic Undead.