Krell was the Klingon responsible for upsetting the balance of power, in 2267, within the culture on Tyree's planet by providing a rival clan of villagers led by Apella with the "advanced technology" of flintlock weapons. He promised Apella that if he cooperated with the Klingons, he would be given power over the planet. In turn, Krell would be able to establish a puppet government with alliance to the Klingon Empire. Krell's mission was foiled when James T. Kirk discovered the plot and armed Tyree's clan with the same weapons. (TOS: "A Private Little War")
Krell was played by actor Ned Romero.
Of particular behind-the-scenes interest is that Krell's darkened complexion and upswept, bushy eyebrows represent a return to the original Klingon makeup design used on Kor and his men in the first season's "Errand of Mercy", and later on Kang, Kahless and all other Klingons in the third season. For some reason, at the beginnning of season two, the Klingons Kras (TOS: "Friday's Child") and Koloth, Korax, and their men (TOS: "The Trouble with Tribbles") were styled with Caucasian skin and Human-style eyebrows.