All too Real achievement in Planescape: Torment and Icewind Dale: Enhanced Editions
Adahn does not exist, but you can bring him into being by talking about him enough. You must mention Adahn, or claim to be him, at least 11 times in order for him to appear. He will manifest in the Smoldering Corpse Bar back in the Hive, so go there and talk to him after mentioning him enough times. Your achievement will unlock at the end of the conversation.
Note: If you keep dragging out the conversation with Adahn, and avoid telling him that he is just a figment of your imagination, then you can get some items and XP out of the deal as well.
It may be worth noting that pretty much every time you lie about Adahn you will move your alignment toward Chaotic.
Here is a list of some of the opportunities to lie about Adahn:
Dead Nations: tell Acaste that your name is Adahn.
Dustman Monument: ask the chanting Dustmen at the entrance to mourn for Adahn.
Dustman Monument: ask Death-of-Names if someone named Adahn has died.
Mortuary: you can tell Dustmen that you are there to see Adahn.
Mortuary: you can tell Soego that your name is Adahn, or that you are there to see Adahn.
Mortuary: you can tell corpse 1094 your name is Adahn using the Stories-Bones-Tell ability.
Ragpickers Square: you can tell Mebbeth that your name is Adahn.
Ragpickers Square: you can tell Ratbone that your name is Adahn.
Trash Warrens: you can tell Bish that your name is Adahn.
Lower Ward: you can tell Hamrys that your name is Adahn.
Lower Ward: you can tell Thorp that your name is Adahn.
Lower Ward: you can tell Karina that your name is Adahn.
Clerks Ward: you can tell Diligence that your name is Adahn.
Brothel for Slaking Intellectual Lusts: you can tell the Modrons that your name is Adahn.
Modron Maze: you can tell Nordom that your name is Adahn.
Curst: you can tell Roberta that your name is Adahn.
