Adversaries are the counterparts to Co-Operators in Bloodborne, and only occur when playing Online. Their goal is only to kill the host of the world, and have a distinctive aura of red or purple shrouding them.
An adversary is another player who has forced their way into another world. They are given full access to their arsenal of weapons and equipment and stats, however. Much like Co-operators, any form of adversary will have their HP reduced by 25%, with the exception of offsetting effects, such as Chalice dungeons, which may heighten this to 50%. Upon the slaying of an adversary, the host will gain Insight and any other hostile players will receive echos along side him. If the Adversary is successful and the host falls, then the message "Prey Slaughtered" will appear and insight will be obtained upon reappearing in their own world.
There are 3 ways to become an Adversary:
First and most common; ring the Sinister Resonant Bell which can be purchased from the Bath Messengers for a single insight once one has bought the Small Resonant Bell. The game will then begin searching for the chime of a sinister bell, if the conditions are right.
The Sinister Bell referred to is not the Sinister Resonant Bell, instead that of the Bell-ringing Woman, who appears upon using the Beckoning Bell, or the Sinister Resonant Bell in an area that the boss still stands.
Second, and often most unexpected; by ringing the Small Resonant Bell with the Hunter rune equipped, you may join the game of a "Blood-Addled" hunter who rung the Beckoning bell as a unique Purple adversary. Multiple Hunters of Hunters can join the same game, but cannot hurt one another.
Third, any hunter who bears the Radiance and rings the Small Resonant Bell and tries to join a bearer of Corruption will instead invade as an Adversary, as the two groups are sworn enemies of one another: The reverse is also true.