Lore:

This sans was after the event of Undertale: Hard Mode, as he went to encounter the human with Papyrus, he died instantly, and sans wills stop him in the genocide run, but failed, and the human was to slash his right eye, and punch him off guard before the eye sockets are cracked and the eye gone run off, but he caught it, his skull was right fractured in the next one, so he carried Papyrus scarf and put the right eye socket, uses as a bandage, he's ready to make a new eye power, his eye was filled with integrity and bravery, and now he's able to hand in hand combat.