Occupy. The parasite can occupy another creature's space and vice versa, and can also move through any opening large enough for a Tiny insect. When the creature hits another one, it gets inside of it, and attacks made against it attempting to hit it can hurt the host as well.

Camouflage. When it is unmoving, the creature is very hard to spot. Investigation and Perception checks to do so are rolled with disadvantage.

Flyby. The Aberrant Parasite doesn’t provoke an opportunity attack when it flies out of an enemy’s reach.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target in the creature' space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half of its hit points or fewer.

If the attack succeeds, the creature enters the -now- host's body. 

Growth: Each day that passes with a parasite inside a body, the victim must roll a DC 15 Constitution saving throw. If it fails, one or more of the following or other possible effects can take place. The infestation can only be ended by consumption of an appropriate medicine that neutralizes the experiment, which can require finding the creators, their journals, or conducting research otherwise as the DM sees appropriate. After every long rest, the DM can also permit a new saving throw, ending the condition on a success and releasing the parasite from the body:

Option 1, Impersonation: on a failure, the parasite reaches the brain and can attempt to control the host, which loses agency. Aware of its memories, it can imitate it quite successfully. DMs can secretly pass a card or information to the player, who can decide to impersonate or temporarily (or permanently) play a new character.

Option 2, Multiplication: after a cellular division, another parasite is bred, and hatches within 1d4 days. It is expelled of the host's body, since only one can live in one same host.

Option 3, bleeding: every hour afterwards, the host/target takes 1d8 piercing damage, for a maximum of 8d8 per day. If a target infested by the parasite ends its turn with 0 hit points, it dies as the creature breaks its chest and goes out in a much greater size. Choose any monster template as the mutated version.