The Genocide ending of Outertale isn't that much different to Undertale's, except for the Sans battle and the final decision with Chara.

Sans battle

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The Megalo­ray, which replaces Gaster Blasters. When fired, the arrow extends in the direction fired.

Sans' battle lasts roughly 50% longer than Undertale's and is split into two equal parts. The first part is normal except for Sans not using Megalorays. The second part introduces an animated background that sports Sans' overworld jacket colors while shifting left and right in a sort-of wave. During this part, Sans uses Megalorays.

Final Decision

If you choose ERASE, the game proceeds as normal. If you choose DO NOT, Chara agrees with your decision and possesses you causing you to meet with Arial, who transforms into Final Arial, the last battle of the Genocide Route. This involves a mechanic where Chara's soul follows your own at the same speed but slightly late, and both yours and Chara's health bars are displayed.

When Final Arial's battle is in its first stage, you must ask for forgiveness repeatedly, decreasing your LV slowly.

At LV 17, Chara becomes aggravated, and the second stage begins, where their SOUL (shaking violently) follows behind yours. They has 99 health, whereas you only have 20. You must lead Chara's soul into Final Arial's attacks until they reach 0 HP, upon which they become too weak to continue possessing you and flees. When this happens, Final Arial reverts back to Arial, and he will talk a bit about game completion before ending the game forcibly.