How to Acquire the Blade of the Immortal in Planescape Tormentby Ty Arthur ; Updated September 22, 2017
The Nameless One is seemingly unable to die, but there is a weapon known as the Blade of the Immortal that is able to kill him. This weapon can be used to get one of the best endings for the game.
The Nameless One must first complete the main quests in the Lower Ward, such as slaying the abishai Grosuk, freeing Trist from slavery and getting Byron Pickit arrested, and convincing the guard Corvus to speak with Karina.
Go to the indoor market in the Lower Ward and speak with Lazlo about portals until he brings up the siege tower that mysteriously appeared in Sigil. Choose the dialog option that states the key to getting into the tower is to not want to get in the tower.
Leave the market and travel to the siege tower. Stand in front by the drawbridge and a dialog option will appear to supress any desire to enter the tower. Choose the option and a portal will appear that leads inside the siege tower.
Speak with the huge metal golem named Coaxmetal in the center of the room. Follow the various dialog options asking about Coaxmetal's purpose and forging weapons. Eventually a dialog option will appear that asks Coaxmetal if immortality is another form of death. Choose this option.
Ask Coaxmetal about forging a weapon that can kill an immortal. Shortly after a dialog option will appear to ask about if an enemy is unseen and attacks from the shadows. Choose this option and Coaxmetal will offer to forge a blade that can kill The Nameless One. He will then give you the Blade of the Immortal.