How to Acquire
Corvo Attano (or Emily Kaldwin in New Game +) must cast the supernatural ability Possession and chain between a human host, a wolfhound or gravehound, a rat, a fish and a bloodfly in any order before the effect ends. Possession and all of its upgrades must be unlocked to be able to chain between hosts, possess a human and last long enough to complete the chain.
The easiest place to chain between a member of each species is during the mission The Royal Conservatory. First enter the Royal Conservatory through the basement and possess the gravehound patrolling on the other side. Go outside and down the stairs on the right then switch host with a rat. Switch host with one of the bloodflies nearby and fly through the large pipes to the fountain outside the building. Switch host with a guard patrolling nearby then switch with one of the fish in the fountain.