The Ten Strong were a group of ten Schirr rebels, led by DeCaster. They were at war with the Earth Empire, committing various terrorist attacks against them. Such as once when they tried to take out the Pentagon sub-router in New Jersey, resulting in many deaths. They were against the Earth Empire as it had annexed (or "repatriated" according to official terminology) their homeworld, which had been renamed Idaho.
They were also partially a cult, with the other Schirrs revering and slavishly devoted to DeCaster because of his use of Morphiean magic, which to them turned him into god-like figure. They celebrated the physical form as part of their religion. In their own eyes they had made themselves physically perfect. When DeCaster, Pallemar and the other Schirrs were incarcerated, they branded with a marking, which was a common practice done to criminals in the days of the Earth Empire. They celebrated these markings as a sign of achievement.
There was a brief power struggle at one point between DeCaster and Pallemar, as Pallemar was worried about DeCaster's increasingly psychopathic behaviour and growing megalomania. He attempted to take control and stop their activities. DeCaster found about this and killed him, tearing his flesh open and bludgeoning holes into body, making him bleed profusely.
They were using Morphiean magical power fight the Empire. (PROSE: Ten Little Aliens)