Marshall Koehler (alias Tyler Brunsen) is the son of Judge Joshua Koehler and his wife Shauna. Koehler was kidnapped in March 1979 at the age of three.
He was long suspected to have been raped and murdered by a pedophile named Roy Lee Dotson. Years later, his father asked Detective Elliot Stabler to interrogate Dotson as to the whereabouts of his remains. It was later discovered that Dotson had nothing to do with his abduction.
Instead, he had been abducted and raised by another pedophile Gregory Brunsen, who sexually abused him throughout his childhood. Koehler obtained a DNA test after a quote he had taught Marshall as a child, "A tyrant will always find a pretext for his tyranny", was spouted at him during Marshall's trial when he was still going by Tyler Brunsen. The test confirmed Tyler was actually Marshall. They are reunited at the end of the episode. (SVU: "Liberties")