The alien race known for assimilating other species into a collective is the Borg. In Star Trek, the Borg are cybernetic beings linked through a shared hive mind called the Collective. They seek what they describe as “perfection” by absorbing useful biological traits, technology, and knowledge from other civilizations.
Assimilation usually involves nanoprobes, microscopic machines that spread through a victim’s bloodstream and alter the body. Cybernetic implants are then added, turning the individual into a Borg drone. The Borg assign species numbers and incorporate assimilated cultures into their expanding network, largely erasing individual identity.
The Borg first became a major threat in The Next Generation episode “Q Who,” after Q sends the Enterprise-D into unexplored space. They later assimilate Captain Jean-Luc Picard and transform him into Locutus of Borg in “The Best of Both Worlds.” That storyline is often confused with the Borg Queen, but the Queen is a later character and not the name of the race or Collective.