Solver

The Solvers were a Hive mind of part-mechanical and part-organic "puzzle boxes". They were described by the Eighth Doctor as highly organised and highly intelligent codebreakers. They were legendary on Gallifrey, resulting in the Doctor's surprise that they even existed.

Enslaved by the Clocksmith, who had come to their world with trinkets which were used to disassemble their queen, gaining control over them. Under his power, the Solvers were instructed to search for the pieces of the Doomsday Chronometer.

Lady Risolva, the queen of the Solvers, sought her people across time and space. When Risolva and the Eighth Doctor finally reached them, in 15th century Prague, the Solvers turned on the Clocksmith, and returned their allegiance to their queen, who told them to disperse the clock throughout history. (AUDIO: The Eighth Piece / The Doomsday Chronometer)

Behind the scenes
The Solvers were voiced by Glen McCready.