A single being created when Dr. T'Ana and chief engineer Andy Billups are merged by a transporter aboard Voyager in the Lower Decks episode Twovix, who then begins fusing the rest of the crew.
T'Illups is the hybrid produced in the Lower Decks fourth-season opener Twovix. The Cerritos is escorting the retired USS Voyager to Earth to become a museum when the same kind of orchid and transporter mishap that once created Tuvix combines the Caitian doctor T'Ana and the engineer Andy Billups into one person.
Captain Freeman looks up how Kathryn Janeway dealt with Tuvix and is appalled to find that she simply ended his existence. T'Illups learns of the precedent too and, determined not to be split, starts merging other officers into fresh hybrids so that the fused crew will outnumber the rest.
The combined crew are eventually blended into one shapeless mass, which D'Vana Tendi and the Vulcan T'Lyn manage to sort back into the original individuals. T'Ana and Billups are restored with the others, and Voyager is delivered to Earth.