During the Reaper assault on Thessia, Vendetta is activated by Commander Shepard. After verifying that it is in a post-Prothean cycle, Vendetta detects the presence of Reapers and attempts to shut down, declaring that the cycle has already reached its "extinction terminus." Shepard urges the VI to wait, demanding to know what the Catalyst is. Vendetta explains that the Crucible is not of Prothean design, and that the Protheans' attempt to use it was sabotaged by a splinter group who argued the Reapers should be controlled rather than destroyed -- much like Cerberus in the present cycle. These separatists were later discovered to be indoctrinated.
Vendetta also mentions that the Protheans have found similar patterns of evolution and dissolution occurred in every galactic cycle. Each path of advancement and conflict happened differently but in fundamentally similar ways -- too similar to be merely coincidences. Whether these similarities include synthetic/organic confrontation, indoctrinated splinter groups, or structure of galactic politics remains unclear. Vendetta infers that the Reapers are merely "servants" of the pattern, not its creators, but cannot identify their true master or motive, only its intention: galactic annihilation. Before Vendetta can identify what and where the Catalyst is, it detects an indoctrinated presence, Kai Leng, and enters a fail-safe mode in an ultimately futile attempt to avoid capture. Kai Leng defeats Shepard with the assistance of an A-61 Mantis Gunship, takes the VI and delivers it to the Illusive Man.
Shepard later learns the true nature of the Catalyst from Vendetta after reaching the Illusive Man's office on Cronos Station. The Catalyst is in fact the Citadel, and it serves to enhance dark energy emissions and coordinate the entire Mass Relay network, thereby extending the Crucible's range. Because the Crucible cannot defeat the Reapers in its current state, the Citadel was incorporated into the design at some point many cycles ago, to use the Reapers' own technology against them. Vendetta was programmed to withhold the information for fear of the Reapers learning of the Catalyst's intended use before the Crucible was completed. Unfortunately, the Illusive Man informed the Reapers of the Catalyst's true nature and they responded accordingly, seizing the station and placing it in orbit of humanity's homeworld, Earth. The Reapers would protect their station at all costs and Vendetta predicts that the probability of accessing it to be remote.