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The earliest you can work on this achievement is during Mission 5, The Royal Conservatory. Take note that this can only be done with Corvo, as Emily does not have the Possession ability. In addition, you must have at least thirteen Runes on hand by the time you reach Mission 5.

Once you reach the outer streets and courtyard of the Royal Conservatory, go ahead and open the character menu and scroll over to the "Powers" tab. Using thirteen Runes, unlock all four Possession skills and then close out of the menu. Your next task will be to pass the Wall of Light at the Conservatory's front entrance and locate the staircase on the far right that leads to the basement and lower yard. In the basement area, you should spot a door that, if you peek through the keyhole, leads to a storage room with a gravehound. To the left of this door is a lower platform infested with rats and a couple bloodflies. There's also a small sewer entrance to the far left that leads to a fountain in the yard where some fish can be found in addition to a single patrolling guard.

After spotting all the hosts you need, create a manual save (for backup in case you mess up), open the door to the storage room, possess the hound using [ Register or Signin to view external links. ] , and then quickly move to the lower platform inside the basement. Now tap [ Register or Signin to view external links. ] to transfer over to a rat and then a bloodfly. From here, fly out of the drainage pipe, transfer over to the human guard, then finish off the cycle by approaching the fountain and possessing a fish. The achievement will unlock after a few seconds of possessing the fish.

Please refer to the video below for a visual guide on how this is done.

NOTE: Feel free to drop a manual backup save before unlocking the necessary skills for this achievement in the event you do not wish to carry on with these skills during your playthrough.

NOTE #2: As of the December 2016 update, the method in the video below no longer works. Possessing a grave hound immediately kills you; you will need to possess the hound last. I suggest going in the following order: human, fish, bloodfly, rat, hound.