"Everyone that had fallen down... ... has woken up. They're all walking around and talking like nothing is wrong. I thought they were goners...?"
– Alphys in ENTRY NUMBER 14 of the True Lab
The Amalgamates are a group of monsters that are the results of the DT experiments carried out by Alphys. As the difference between human and monster SOULs is the amount of Determination contained within them, the experiments were an attempt to add the missing aspect into the monster SOULs so that they could be used to break the barrier.
The monsters used in this experiment were ones that had 'fallen down', assumed to mean that they had fallen into a coma or a similar near death state. The added Determination was enough to allow the injected monsters to regain consciousness, which was not the intended effect. Unfortunately for the injected monsters, Determination is an unstable chemical which requires a minimum amount of matter to prevent damage to the physical form. As monsters are mostly made of dust and magic, rather than physical matter like humans, their bodies became unstable and essentially melted into the bodies of other monsters. Either due to the amount of Determination within them, or the fact that they are technically already dead, they are unkillable.
Amalgamates all are colourless in the overworld. Their animation and attack sprites are delibrately designed to be unsettling and break previously established rules. They often also use the attack patterns or sprites from the other monsters they are made of but with creepy twists. With the exception of Endogeny, they look different between the overworld and the battle sprite. They may have the ability to change shape as Lemonbread is shown to. Excludeing Memoryhead, the amalgamates' battle sprites are made of components of the monsters contained within them and serve as clues to what ACTs should be used to SPARE them.
List of AUs having the Amalgamates