In keeping with the Pokemon tradition, Spiritomb is another Pokemon with a horrifying backstory. It is a Generation IV ghost type Pokemon-- #442 in the Pokedex-- that looks like a malevolent spirit entity (which it is) coming from a small jar. The jar is actually an item called an Odd Keystone, which is an in-game item that the player finds and the first step for summoning a Spiritomb.
Spiritomb is made, literally, out of 108 cursed spirits who have been bound into the Odd Keystone for their sins. They are aggressive, destructive, and full of rage. Their secondary type-description is the "Forbidden Pokemon" for a reason.
To gain a Spiritomb, the player must:
1. Acquire an Odd Keystone. One can be given to the player by a Blackbelt character in Hearthome City, they can dig one out in the Sinnoh Underground, or there's one on the ground somewhere in Twinleaf Town.
2. Take the Odd Keystone to the Hallowed Tower on Route 209 and leave it there.
3. Go to the Underground again and talk to 32 other players. Alternatively, you can talk to the same other player 32 times while digging or leaving the Underground between each dialogue. You must have the other action between each dialogue, or else it won't count.
4. Check on the Odd Keystone. When it's ready, a Spiritomb will appear and start a battle with the player.
5. CATCH IT.
The Diamond, Pearl, and Platimun Pokedex entries for it are:
"A Pokémon that was formed by 108 spirits. It is bound to a fissure in an Odd Keystone."
"It was bound to a fissure in an Odd Keystone as punishment for misdeeds 500 years ago."
"Its constant mischief and misdeeds resulted in it being bound to an Odd Keystone by a mysterious spell."