The Architects were an ancient civilization that once existed within the galaxy.

They were known to have been a race of mortals who created both the Guardian of Forever and the Devil's Heart. However, they died at some point and there was no one to guide the path of the Devil's Heart seed to its true destination. (TNG novel: The Devil's Heart)

The novel Preserver suggests that the Preservers may have created the Guardian which may mean the Architects were another name for them but this is speculation.
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