SPOILER WARNING: This article contains major spoilers for "Ahimsa," the latest episode of Supergirl.
In "Ahisma," the latest episode of Supergirl, fans bid goodbye to Mercy and Otis Graves. The sibling duo were killed in a botched terrorist attack that they planned to pin on aliens.
Over the course of the episode, J'onn discovered Mercy and Otis were controlling his friend Fiona Byrne with an alien parasite. While she was under their thumb, Fiona involuntarily used her telepathy to force other aliens to commit violent acts, like attacking a police station. After a suggestion from Agent Liberty, though, Mercy and Otis switched their tactics and set their sights on a family fair passing through National City.
At the fair, Mercy and Otis used Fiona to persuade their alien captives to attack families. A battle with Supergirl and the D.E.O. ensued, during which time Lena Luthor and Brainiac-5 cleared the Kryptonite from the air. As soon as she could survive without the suit, Supergirl got the upper hand on Mercy and Otis.
As Alex moved to arrest them, Mercy and Otis were both shot in the chest with bone-like darts. Shocked, Alex and Supergirl turned to see that one of the mind-controlled aliens -- free of Fiona's hold -- had turned on his captors and killed them. Former D.E.O. Agent Jensen later confirmed their deaths during a discussion with Agent Liberty.
Mercy and Otis were introduced just three episodes ago in the Season 4 premiere. They debuted as an anti-alien terrorists who orchestrated an attack that outed President Marsdin as an alien to the pubic and sparked nationwide chaos. As a flashback revealed, they personally recruited Ben Lockwood to be their figurehead Agent Liberty. Per the episode's conclusion, it seems that Agent Liberty is poised to take over the movement.
Airing Sundays at 8 pm ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen, and Chyler Leigh as Alex Danvers.