The Mydacai
Mydacai are a species that is born between the universes. They have no substantial body and appear like smoke that moves within a certain area. That is partly the case because they can never entirely exist in *this* world. They can only enter it by possessing a host. That host is usually the Caimanae. The Mydacai are not parasites though, in fact they share a symbiotic relationship.
Born in the gap between universes they're left with no existent conscience. They're merely matter floating in an empty space. They can never really enter another world, simply vanishing if they would stay too long without a host and not being able to enter at all without being summoned.
That's where the Caimanae become important. They pull the Mydacai into their world and a chosen individual who came to full maturity.

The Caimanae
As they're the host of the Mydacai it's very important to understand their background.
The Caimanae are born warriors, they have been waging war for centuries and for them peace shall never exist. It's what they believe in and they're rather traditional and narrow-minded when it comes to that.
Their appearance is a mixture of man and reptile. I tried to draw it (link). They're strong and very fast; they can walk on two legs and also have a tail to keep their balance and sweep their enemies off their feet. Their scales are very hard to penetrate and therefore serve as an in-built armor.
Despite that they're only averagely intelligent. They still believe in shamans, magic, ghosts and rituals. All this is based on fate instead of a god.
The Caimanae live on the planet Caima. This planet is only separated by a fine veil from other existences. Long ago they started to use that to their advantage, summoning ghosts and spirits they later named the Mydacai. In addition they also use the technology that sometimes transfers through the veil and through that became a rather advanced planet without ever giving up their beliefs of war and ghosts. Only the belief in Fate loosened.

The Merging of the Minds
That's what the ritual is called. When a Caimanae takes in a Mydacai as a first host he won't really notice any difference. It only becomes interesting when the first host dies and the Mydacai is forced to leave his body. The Mydacai will quickly need to find a new host or he'll vanish. All memories of an individual are transferred to the next host and like this both species profit. The Mydacai stays alive and the Caimanae retain knowledge of centuries of war strategy.
Through continuous merging a Mydacai will gain an own personality, an own conscience. They become a small whisper in the host's head, but it's not seen as unnatural as they whisper secrets of survival and victory. Much later they gain control of the host's muscles and their head, they become dominate because they're the stronger personality, the stronger mind.

Creating
As of now, you're not allowed to create neither of them. If you do have an idea though, just contact me and maybe I'll agree! : D