Warning: minor spoilers ahead for tonight's episode of Krypton, titled "The Rankless Initiative."
Salmon, who played Walter Steele on Arrow, is playing a Black Zero terrorist seen in hologram form during tonight's episode. He may or may not be Black Zero leader Jax-Ur, who Primus Jayna Zod names, and says that there is no credible photograph of in their files.
So far in the series, little is known about Black Zero. The main antagonists of the corrupt theocracy run by the Voice of Rao, the group are hardly the hope-fueled revolutionaries promised by Seg-El's opening monologue in the pilot. Instead, as has been rightly pointed out by Daron Vex, the terrorist who made an attempt on the life of the Voice of Rao in the pilot was just one of a number of attackers, many of whom have targeted civilian targets.
We do not know who actually serves Black Zero, only that their group was used as a cover for the actual activities that Seg's parents undertook to protect the Fortress of Solitude.
In the comics, Black Zero is a rather...fluid...concept. The first Black Zero, created in 1968, was a supervillain who was directly responsible for the destruction of Krypton. There was also The Black Zero Organization, a Kryptonian terrorist organization responsible for designing and unleashing the Black Zero Virus, which attacked the technology in Superman's Fortress of Solitude. An intelligent computer virus, Black Zero was dormant in the Kryptonian technology used to create the Fortress of Solitude. After being inadvertently activated, Black Zero ran Superman and the Legion through a warped version of Kryptonian history, intended to finish with the destruction of Earth. The organization itself was a group opposed to the common Kryptonian practice of keeping clones as slaves; this began the War of Clone Rights, which lasted for a millennium. In their final act, Black Zero detonated a device that would later become known as the Destroyer. In essence, this device functioned as a giant nuclear gun, that fired a concentrated, sustained burst of nuclear energy directly into Krypton's core. Though subsequently destroyed by an ancestor of Jor-El himself, the Destroyer's effect would later be fully realized; It would cause a chain reaction deep within Krypton that would later obliterate the planet (a recurring motif).
There was also a Black Zero (one also tied to clones) related to the '90s version of Superboy. An evil clone of Superboy himself, Black Zero appeared a handful of times, terrorizing Superboy and the people he was protecting. Most recently, the name "Black Zero" was used by an elite Kryptonian military force, under the command of Ursa. The unit, except for Ursa, was killed attempting to defend Kandor from Brainiac. A version of this iteration appeared on the long-running CW series Smallville, as well. In the show, Black Zero was a Kryptonian rebel group that started a civil war with the Ruling Council of the planet Krypton. In one of their most famous battles, Black Zero held their battlefield at the city of Kandor, with Major Zod leading the charge against the rebellion.
Then, in Man of Steel, Black Zero was the name of Zod's ship, which hovered over Metropolis while he and his Phantom Zone criminals battled Superman.
Besides his time on Arrow, Salmon is best known to genre fans as an actor recognizable from movies like Alien vs. Predator, Resident Evil, and The World is Not Enough. Recently, he appeared in 24: Live Another Day and the Ryan Reynolds/Gal Gadot movie Criminal.
Krypton airs on Wednesday nights at 10 p.m. ET/ 9CT on SYFY.