The Genocide Route is one of the main routes in Undertale Yellow.
How to do Edit
Like in Undertale, the player has to start killing monsters in order to walk the Genocide Route. They have to kill every monster in each area until encounters no longer occur.
However, unlike Undertale, instead of encounters being replaced with the message "But nobody came" an encounter balloon appears with a red "X" replacing the exclamation mark, and the encounter does not occur. From this point forward, all encounters will be replaced with the red "X" until a new room is entered (unless the kill count has been exceeded, then the "But nobody came" message is shown, and a slowed down, low pitched version of Enemy Retreating plays.)
Dark Ruins Edit
At the start, nothing very relevant happens. Dalv's monologues remain the same, enemy encounters continue triggering, and puzzles still have to be fully solved. However, if the player continues their Genocide Route until they reach the room that they encounter Crispy Scroll, the normal battle music (Enemy Approaching Yellow) will be replaced by Enemy Retreating.
If the player manages to exhaust the kill counter in the Dark Ruins, Flowey will appear after killing the last monster, saying that Clover "is not for that mercy thing", but that he's their friend, and that real friends don't judge each other. Then the music played in the overworld will slow down and lower pitch.
From the start, Dalv's behavior is unchanged and he will still recite the same monologues, seeming unaware of the Clover's genocidal actions. Even after Enemy Retreating and Flowey's appearance, his final monologue before his battle will remain the same.
However, when the battle starts, his dialogue will be entirely different. He will mention that he despite his desire to be left alone, that he noticed dust lying all over the Dark Ruins, and that nobody comes to his house anymore. He admits that he wished for everyone to be gone and that he should thank Clover for their actions, only to realize that he now misses the late monsters. Dalv will then mention how he regrets never trying to befriend the monsters in the Dark Ruins and begins to wonder if his wish for them to disappear was somehow granted, making Clover a figment of his imagination, and the entire battle a dream. Upon meeting his death, however, he realizes that Clover is indeed real and that when he wakes up, Clover will still be there. He realizes that it's likely he just won't wake up at all, before dying as he turns to dust.
However, if he is spared, he will become extremely angry, saying that Clover only spared him to make him suffer more, and will stay in his room until the player leaves for Snowdin through his house.
Dalv's House Edit
If Dalv is killed, they will have access to the entire house, including Dalv's previously inaccessible room. The descriptions of the items will be much crueler, and if the wardrobe is opened, Pops's helium will be sucked in by Clover, making them sound funny for a while. Just like in the Neutral Route (if Dalv is dead), Flowey will break the planks for Clover to pass.
If the protagonist spares Dalv, he will be in his room all the time. The description for the items will be the same as on the Neutral Route, and Flowey will open the door for Clover to pass.