Man from Rubizhne, whom Ukraine suspects of treason, has been sentenced to 18 years in prison in Russia on charges of "terrorism"

The Southern District Military Court in Rostov-on-Don sentenced Roman Serhiienko, a native of Rubizhne in the Luhansk region, to 18 years in a maximum-security prison. In Russia, he was charged under "terrorism" laws, while in Ukraine he is a defendant in a treason case.
According to Censor.NET, this was reported by the Russian publication "Mediazona."
What was the Ukrainian accused of in Russia?
Serhiienko was charged with participating in a terrorist group and training in terrorist activities. The details of the case are unknown, as the court heard it in closed session; however, the charges are typical for cases against prisoners from Ukrainian units that are recognized in Russia as "terrorist organizations"—"Azov" and "Aidar."
It is known that Serhiienko is from the city of Rubizhne in the Luhansk region, which Russian troops captured in the spring of 2022. Sergienko was detained in April 2022; he is being held in a pretrial detention center in Luhansk and heard his sentence via video conference.
The anonymous project "Nemesis," which publishes information on Ukrainian prisoners of war, lists Roman Serhiienko as a member of the DIU. Meanwhile, the Ukrainian website "Myrotvorets" refers to him as an "accomplice of the Russian occupiers."
What is the convicted person accused of in Ukraine?
The website of the Prosecutor General's Office of Ukraine has published the charges against Serhiienko under Part 2 of Article 111 of the Criminal Code of Ukraine (abuse of power under martial law).
It states that, under circumstances that have not been established, but "no later than March 2022," Serhiienko, while in occupied Rubizhne, voluntarily joined the Russian army, as confirmed by a video showing him wearing a Russian uniform and holding a Russian flag. The Prosecutor General’s Office of Ukraine does not provide any further details.