The Marshal was secretly under the Shadow's influence, having been hypnotised. He contacted the Shadow through an object that resembled a skull. At his master's behest, the Marshal launched a final strike against Zeos, unaware that Mentalis had started its self-destruct program, which would also destroy Atrios.
The Fourth Doctor placed him and his ship in a time loop using the Key to Time. However, this began to gradually weaken. The Doctor redirected the Marshal's attack against the Third planet, destroying it and apparently killing the Shadow.