It is not known to evolve into or from any other Pokémon.
Although lacking a physical body, Spiritomb projects its appearance out of a small rock called the Odd Keystone. The rock is trapezoidal in shape and has two small dots. There is a crack running down the middle, splitting in two near the bottom. The crack in combination with the two dots appears to form a frowning face. Spiritomb's projected appearance is an ethereal composition of swirling purple fog. Its face is set in the middle of the fog and contains green, crescent eyes that are connected to its jagged mouth. Its pupils are composed of a spiral and it is sometimes only seen with one. Swirling around its face are green orbs with yellow centers.
Spiritomb is formed by the collection of 108 spirits being trapped in an Odd Keystone due to misdeeds. Some Spiritomb are up to 500 years old. As seen in the anime, Spiritomb can be just as aggressive and destructive as the spirits that it is composed of. It will lash out at anyone with little thought, often exercising short sightedness in its pursuit of vengeance.
Prior to the introduction of the Fairy type in Generation VI, Spiritomb was one of only two Pokémon that had no weakness (without the aid of an Ability) along with Sableye. As this occurred without the aid of an Ability, it was commonly hacked to have the ability Wonder Guard, making it immune to all direct damage. This hack is commonly referred to as Wondertomb.
Spiritomb seems to be based on a Jibakurei, a spirit bound to a single place. It might also be based on a Buddhist tradition performed on New Year's Eve in Japan. A bell is rung 108 times to chase away the 108 temptations in order to achieve Nirvana. The wisps of spirits surrounding its face also resemble a set of prayer beads, a possible reference to Japa.
Spiritomb may also be based on Water Margin, one of the Classic Chinese Novels, which is also very well known in Japan. In the first chapter of the novel, a foolish army marshal orders the removal of a stone monument which had been guarded by monks for generations. Doing so releases the spirits of 108 demons sealed therein. These 108 souls, having atoned for their sins, become the 108 Stars of Destiny. They are reincarnated centuries later as 108 outlaws, the heroes of the novel.
Spiritomb is a combination of spirit and tomb.
Mikaruge may be a combination of 甕 mika (jar or urn, referring to it being a container of souls) and 軽石 karuishi (pumice) or 御影石 mikageishi (granite; both refer to its base, the Odd Keystone).
In other languages
From 甕 mika (jar or urn, referring to it being a container of souls) and 軽石 karuishi (pumice) or 御影石 mikageishi (granite)