When their planet was attacked by a joint Borg/Cybermen assault force, they were easily overwhelmed. Their prime minister ordered a planetary evacuation, but not before much of the planet, and many of the species, were taken by their assailants. (COMIC: Assimilation²)
Behind the scenes
A Deltan by the name of Lieutenant Ilia featured prominently in the storyline of Star Trek: The Motion Picture (as the victim of V'Ger's first meeting with the Enterprise), but the species has otherwise gone un-used by most Star Trek productions. None of the Deltan telepathic or psycho-sexual powers were explored in the first issue of Assimilation². Thus, so far as the DWU is concerned, they're just "bald humanoids".
- Deltan at Memory Alpha, the Star Trek canon wiki
- Deltan at Memory Beta, the wiki that covers all licensed Star Trek works