A symbiotic lifeform permanently bonds with its host and radically transforms the host organism.
Imagine a symbiotic life form like the [Klyntar](http://marvel .wikia.com/wiki/Klyntar_(Race)) species from Marvel: a creature made up of an amorphous mass of cells and are, as a whole, sentient.
The species would have the following additional characteristics:
- they could only form one permanent bond with a host
- they could evolve the host in any fashion or change its form at will
- it would merge with the host at a molecular/genetic level and preserve its consciousness via adding its cognition center to the host body permanently along with decentralizing the host nervous system to speed up cognition via entangling the neurons together so the host will not die if shot where previous brain was
- the host would receive enhanced healing, strength, speed, intelligence, and depending on the environment that the host lives in the symbiote will adapt the host beyond current capability's.
- the symbiote could communicate with the host, allow them to support one another emotionally and physically during their nearly infinite lifespan.
- In such a life-span that while technically immortal the host and symbiote would not survive forever due to war, violence, etc
- symbiotes would make people invulnerable to disease, poison, radioactivity, and cancer (note that anything else that could detrimentally harm the host body would also be mitigated through the enhanced abilities)
- the symbiote could only override the will of the host if either knocked unconscious (accident or combat) to save the host from harm or with the hosts explicit permission
- the bonding would only be consensual or through birth (i.e. hosts and symbiote would mate)(the infants would grow up at a decelerated rate i.e. 1 standard year of growth would be doubled for the infant (36-40 years to adult-hood))
-they would be two separate entities with distinct personalities but they would each give the other suggestions as to what or what not to do
- the bonding to a degree would not be personal choice but rather a gut feeling and the use of what would be best for both rather than one (i.e. telepathic subconscious decision)
the host would gain the innate abilities of the symbiote (i.e. telepathy, a morph-able body and any other abilities that were only innate to that specific symbiote)
the bonding process could last a while (i.e. 1-2 days all the way up to a year or two in rare cases ) so the bodies have time to merge and develop
Most sci-fi focuses on the physical side of such a bond (e.x. Venom, Carnage) or parasitism involving possession or overpowering of the will in such a theme as this it is stated to be an extreme case of parasitism. Not much is is written about the other route and what will happen with extreme symbiosis between two organisms and the psychological effects such a bond would have on the host and symbiotic organism in such a bond.
Given stated examples what long term psychological effects could happen to a given host after a log period of time?