The true lab was created by Alphys and this is the place she most likely built Mettaton. The true lab is a lot like the Undertale concept but more violent. The walls are bloody red and the other stuff can be brown, black, and other dark colors. The true lab holds the amalgamates who try to kill Frisk. Unlike in Undertale, Alphys does not save Frisk, but Flowey does. The amalgamates are much more vicious and are constantly attempting to kill Frisk, re-appearing throughout the Lab until Flowey wards them off.
Monsters like Endogeny and Lemon Bread are most prominent and most vicious, whilst Reaper Bird often keeps to itself, speaking horrible threats and ominous predictions, whilst Snowdrakes mother has never been found. Anyone who has even seen a glimpse of Memory Head has been forgotten.
Only a fool would attempt to enter such a twisted place...
Alphys reveals through her Lab documents that she created the Amalgamates for pure pleasure, and manipulated the families, purposely turning the fallen monsters into horrible beings with no feelings of compassion, purposely letting them scream and fear as the merged together, and purposely leaving them alone. Very alone.
After Alphys decided to show you the insides of the True Lab, she desperately locks you inside in hope that the Amalgamates will tear you bit to bit. With enough interaction, they slowly side with you, and you can then, with the united power of the fearsome but lonely Amalgmates, break out of the True Lab. The Amalgamates then confront Alphys, who desperately tells you that if the Amalgamates kill her, they'll wreak havoc throughout the Underground.In order to complete a True Pacifist Route, you must save Alphys from the Amalgamates, causing her to truly accept you as friend. However, it's still possible on a Neutral Route to let the beasts viciously maul Alphys, tearing her from piece to piece.