Spoilers are precisely defined here. Rules vary by the story's medium. Info from television stories can't be added here until after the top or bottom of the hour, British time, closest to the end credits roll on BBC One. Therefore, fans in the Americas who are sensitive to spoilers should avoid Tardis on Sundays until they've seen the episode.
Even before the outbreak of the "War in Heaven", Faction Paradox was regarded as the most unpredictable (and opportunistic) of the time-active powers. Aware of the precarious nature of history -- but under no obligation to protect it -- while the other Great Houses were still attempting to uphold a "universal order", the Faction was following its own, far more ambiguous, protocols. Ruthless, secretive and at times difficult to understand, it's hardly surprising that the Faction should have eventually found itself under siege from its rival powers...
The Sontarans are used as shock troops to pave the way for the invasion of the Eleven-Day Empire by other interested parties. Ironically, they themselves had earlier used the Vardans for the same purpose. (TV: The Invasion of Time)
For the purposes of this list, a "Sontaran story" is one in which one living, authentic Sontaran plays a part within the confines of the story, outside of flashbacks to previous stories and cliffhangers that lead into the following story. For this reason, stories such as The Halloween Apocalypse and Survivors of the Flux are not counted due to the Sontarans' appearance being defined by their importance to following stories. Stories like The End of Time and The Time of the Doctor are also not included due to the Sontarans' appearance being no consequence to the plot of the story. Stories featuring Strax of the Paternoster Gang are only included on the Paternoster Gang story template.