Spiritomb is a Ghost/Dark-type Pokémon that is bound to a physical object known as a Keystone. Legend holds that a Spiritomb is formed of numerous spirits that were bound to an Odd Keystone as punishment for their misdeeds.
Traditionally, the Keystone is an actual melon-sized stone with a deep fissure. A Spiritomb emerges from this crack in the rock, and is capable of retreating entirely into its Keystone. A Spiritomb is permanently bound to this Keystone, and even when manifested outside, it maintains contact with its Keystone; a Spiritomb cannot leave or break contact with the Keystone. This can create some difficulties with locomotion, mainly being that it can make it quite difficult for a Spiritomb to get around. It must move its Keystone around itself by dragging, rolling or hopping with it -- which isn't terribly effective or fast, and strenuous to boot -- or a Spiritomb can convince someone else to move its stone for it.
A Spiritomb-Morph is not always formed of exactly 108 spirits bound together -- but they generally are more than one spirit, intertwined into and functioning as a single being, whether they were a Spiritomb that was 'morphed or the manifestation of a dead Pokemorph. A Keystone is not always necessarily a stone, but it is generally a sturdy object of some sort with some kind of obvious, physical damage, and about the size of a traditional Odd Keystone (about the size of a pomelo to a basketball).
A Spiritomb-Morph is as bound to its Keystone as a non-morph Spiritomb, and a 'morph may also similarly retreat into its Keystone entirely. When entirely retreated into the Keystone, it appears as a mundane object -- a Spiritomb's Dark-typing makes it immune to Psychic-types' sensory abilities, though Pokemorphs sensitive to auras (such as Lucario) may be able to sense something 'amiss' with the object.
The Keystone can be a weak point for a Spiritomb-Morph. The attachment of a Spiritomb to any object, however, imbues the Keystone with a preternatural strength, and it is quite difficult to destroy a Keystone; there are other methods to dispatch Ghosts. While the Keystone is usually difficult to inflict damage to, much less destroy, destroying the Keystone CAN be done. Damaging a Keystone is debilitating for the attached Spiritomb, and while Ghosts cannot exactly die again, managing to demolish a Keystone guarantees the end of the Spiritomb.
A Spiritomb-Morph has the advantage that they can simply pick up their own Keystone and carry it with them wherever they wish to go. Some Spiritomb-Morphs may even incorporate their Keystone into a specialised pocket in their clothing, or even into their own manifested body. While a Spiritomb themselves can still "phase", the Keystone cannot under any circumstances, and this can be a slight hindrance; generally, though, a Spiritomb-Morph will just have to settle for opening doors before entering or exiting a building.