Yarka was a Bajoran and member of the Vedek Assembly until the year 2371. According to official record, Yarka was defrocked for "teachings not in keeping with the Bajoran faith." Unofficially, according to information Constable Odo received, he was stripped of his title due to leading protests against the peace treaty with Cardassia which the Vedek Assembly endorsed.
In 2371, two months after Yarka was defrocked, several Cardassian scientists came to the Bajoran sector in an attempt to establish permanent communications to the Gamma Quadrant through the Bajoran wormhole. Yarka became convinced that Trakor's Third Prophecy was coming true and that the Celestial Temple would be destroyed. He urged Commander Benjamin Sisko not to assist the Cardassians in this venture, though Sisko ultimately chose not to follow Yarka's warnings.
After the wormhole was not destroyed, Yarka concluded that he had misinterpreted the prophecy and apologized to Sisko for allowing his mistrust of the Cardassians to blind him to the Prophets' will. He did, however, add that there were signs that Trakor's Fourth Prophecy would soon come to pass. (DS9: "Destiny")
Background information Edit
The script for "Destiny" states the pronunciation of Yarka's name as "YAR-kuh." Furthermore, it describes him as "forty or fifty, with a kind face and a gentle smile." This would put his birthdate sometime between 2321 and 2331. 
The character was named for Jarka Burian, one of co-writer David S. Cohen's favorite theater professors at SUNY Albany. (Screen Plays: How 25 Scripts Made it to a Theater Near You - For Better or Worse, pp. 9-10)
In a 2010 interview, Erick Avari recalled some of his personal experiences playing Yarka; "[It] was a very talky piece and I played a very serious character. We worked 12 to 16-hour days, which Star Trek was famous for. So it was a grind, and I tend to get really giggly when I'm really tired, and if I get the giggles I'm in deep trouble and I know it. Tears start running down your face, your make-up starts to smear and then you can't get through your lines. No one thinks it's funny and you know that. Nina Kraft was my makeup woman for this episode, and she got me going in-between takes. At one point it was one o'clock in the morning and the two of us were just laughing like silly schoolgirls. The director then said, 'Places everyone,' and I thought to myself, 'OK, I've got to straighten up.' I had this long, wordy scene that I was supposed to be very serious in. It was about doom and gloom and prophecies to come, and all I remember is just praying that I would keep a straight face through the take. So that was a fun episode." 
In the Pocket DS9 novel Objective: Bajor, Yarka visits Sisko to warn him of Kai Winn's attempt to cast herself as the protector of Bajor as predicted in the Third Prophecy of Andaki. Yarka believes it is Sisko who the prophecy refers to and would save Bajor from the Hive, who were threatening to use the planet as an energy source. Yarka later helps Sisko to procure one of the Bajoran Orbs in an attempt to contact the Prophets and learn the truth about the voyage of the Hive starship.