Undertale Yellow Gameplay Walkthrough DECIBAT BOSS - Pacifist Playthrough Lets play (PC) Welcome to UNDERTALE YELLOW. In this fan made prequel to the RPG #UNDERTALE, you control a human who falls underground into the world of monsters... but will you make it out? Welcome everyone I am Purpleoak and in this video of Undertale Yellow I show you the full playthrough of the Decibat boss and how to do it the pacifist way. You will also hear the music theme that plays with this boss. Undertale Yellow revolves around Clover, the seventh human who fell through the barrier on Mount Ebott into the Underground, and the owner of the Yellow soul from Undertale. The game will feature a cast of mostly new characters, while a few original characters will make a return.