To become a demon in Weak Legacy 2, the player must start as a human and then go to a specific NPC named Douma located in the snowy area to the far right of the Starter Village. You need to interact with Douma and pay him 1,000 Yen to transform into a demon. If you change your mind, you can reset your race back to human by speaking to the Reset Race NPC in Little Village, although this costs Robux unless you have a race reset ticket from codes. Demons in the game use Blood Demon Arts as their special abilities with various skills learned from different BDA trainers.

Step-by-step to become a Demon:

• Start the game as a human.
• Travel to the snowy area located to the far right of the Starter Village.
• Find and talk to the Douma NPC.
• Pay 1,000 Yen to Douma to become a Demon.

Additional info:

• Demons use Blood Demon Arts with unique movesets.
• Resetting your demon race can be done at the Reset Race NPC in Little Village.
• Race reset typically costs 120 Robux unless you get tickets from codes.

This summary provides the complete method to become a demon in Weak Legacy 2 along with additional gameplay details related to demon skills and race resetting options.