These are the SOULs of the Six Humans fused with those of several monsters. Speaking of which, the Combined Souls are a new element to the game.
Papyrus also serves as the final boss if the Player has not found the Combined Souls. Papyrus's battle is moved up to after Sans and the updated Asgore fight, with his fight taking inspiration from several 'Genocide Papyrus' fights and a few incarnations of 'Disbelief Papyrus'. Mettaton is unique in that he's first fought as Mettaton EX as in the Neutral Route, after which the new Mettaton NEO fight proceeds. Virtually every boss in the game is altered somehow, though Sans and Undyne the Undying have the least changes.
Some NPCs the player never fought are now battled, such as Alphys, Grillby (both fights taking inspiration from Alphys NEO and the fanmade Grillby fight, respectively), the Snowdin Shopkeeper and Bratty & Catty (the shopkeepers next to the MTT Resort). Whereas in the original Genocide Route, Undyne the Undying and Sans were the only ones that put up a real fight while all others did nothing and died in one hit, in True Genocide this applies to all bosses. The DLC takes many elements from various fanmade Undertale fights.
