Poltergeists are a species of non-organic lifeform - usually, a spirit - that has become associated with an object. These can be made naturally or artificially. Generally, a poltergeist has an anthromorphic "presentation." However, these are not different "species" of poltergeist, as the presentation is not the spirit's true body (this is the object they are associated with) and the spirit themselves chooses their appearance.
Due to their nature, poltergeists have naturally strong connection to æther.
Natural Poltergeists
Sometimes, a spirit naturally finds itself drawn to a physical item. This can happen even if there are not any mana crystals, though the spirit's own power may be used to create some on the object.
Natural, or wild, poltergeists are somewhat less common than artificially created ones, since the soulsmith can facilitate the process in a way that is rare in the wild. However, this is still a known event to occur.
It is believed that items with more emotional value to the people around the object are more likely to be able to turn into poltergeists - after all, such affects æther. However, this is not a complete picture of the mechanics. For example, despite many spirits lingering in the capital of the Northern Crown, no important buildings, which would have received a lot of æther from the inhabitants hoping the castle can protect them, became poltergeists. One explanation proferred by Sérilennite scholars is that only smaller objects can become poltergeists, but Ramulacra's Golem seems to disprove this notion, as it is a poltergeist of comparable size to a house.
Soulsmithing
Several centuries ago, the technology of soulsmithing was invented by Makrikyria. Soulsmithing is the process by which one can create a poltergeist, associating a spirit with an object and giving it life and magic. When applied to a tool of war, this is called a "spirit weapon."
Spirit weapons gave Makrikyria and those that they gave the technology distinct advantages in combat, resulting in many of the kingdoms of old being replaced, such as the Northern Crown, who was conquered by Sérilenn due to the latter having significantly more spirit weapons.
Spirit weapons come in many different forms. For example, the Northern Crown's legendary sword Corazon, the ancient scythe Marzipan, and the giant shield Mallow are all such poltergeists. They have the soulsmithing in common, however - all spirit weapons, regardless of type, have a spirit within them that can appear and that imbues the item with magical powers.
Creating poltergeists manually generally requires special mana crystals found most readily in Makrikyria, which is why they have stood the test of time and survive to the present. While as stated before, this is not a requirement, it makes the process significantly more consistent.