Thrax was a male Cardassian security officer who lived during the mid-24th century. During the 2360s, he was stationed aboard Terok Nor during the Occupation of Bajor, where he was in charge of security on the Promenade. He was eventually replaced by Odo around 2365.
In 2373, Odo, Jadzia Dax, Benjamin Sisko, and Elim Garak experienced a shared telepathic reality in which Thrax arrested them for the attempted murder of Dukat in 2367. They assumed the identities of three Bajorans that were executed for the attempted murder. Even though Thrax could have investigated further and cleared the men, he did not. As the telepathic reality played out it was discovered that Odo, not Thrax, was the one who let the prisoners die. (DS9: "Things Past")
Thrax was played by Kurtwood Smith.
Smith commented: "[Thrax] was a character within a character, and he was also a mirror of Rene's character from a different time. I loved the depth of it. I always liked the size of the characters on Star Trek. It reminds me of when I used to do a lot of Shakespeare, back when I was first coming up. It has that feel about it, you know, because you've got all this stuff on and you're dealing with enhanced language. They’re just very fun, complicated characters'". 
The character appears in the "Foundlings" short story in the Prophecy and Change anthology. He later appeared in the Star Trek: Terok Nor novels Night of the Wolves and Dawn of the Eagles, where he was given the full name Thrax Sa'kat.
The Star Trek Customizable Card Game uses a modified picture of Thrax for the character Ghoren.