An Assimilation Monster is a type of Monster Card introduced by HelixReactor. They are stored in the Extra Deck.
Color-wise, Assimilation Cards have a bordeaux border, with a crimson stripe in the middle. In a similar fashion to Pendulum Monsters, their effect text box is split into two; the top box is named Host Effect, and the bottom box is the Standard Effect.
When an Assimilation Monster is Assimilation Summoned, a Host Card will be attached to them. Host Cards are not meant to be detached as effect costs, although it is possible to attach and detach Host Cards from an Assimilation Monster via card effects. However, an Assimilation Monster cannot have more than one Host attached to it at a time.
In addition to their Standard Effect, Assimilation Monsters have an additional Host Effect that depends of its Host's card type. Typically, each Assimilation Monster Card has a Host Effect for each of its possible types of Hosts. But it is also possible for some Host Effects to be shared by multiple types of Hosts. It is also possible for some types of Hosts to not have their own Host Effect. Likewise, some Assimilation Monsters may not have any Host Effect at all.