Merren Vethar is a former Veden highprince, and was killed during the Veden civil war by the party. He murdered Theren’s father at the beginning of the Veden civil war, took his shardblade, and took his highprincedom.
The party has interacted with Merren several times; the first time he tried to arrest the party at the Vedenar docks. He was scared off by Nucko the Button Thief who used an illusion to imitate Theren’s father which caused Merren to run away.
Merren has claimed that Theren and his father were “oppressive” to him and his family, and ironically Theren doesn’t seem to remember him.
Merren has asked King Taravangian for an alliance with his house in an attempt to solidify his power.
Merren eventually made his way back to Vedenar to help his allies in the civil war. The party found him and absolutely wrecked him. Kael trapped him in a sinkhole and the party wailed on him until Tink, thinking that Merren was Chungledown Bim, made the killing blow winning Merren’s (Theren’s father’s) shardblade.