King takes Bishop
Stephen had grabbed the throne in 1135 (in a rare burst of decisiveness), Roger accepted the situation, and continued as Chief Justiciar to the new King.
But Stephen quickly lost his grip on events. He alienated his supporters and showed weakness to his foes. Neither Roger nor his family made any overt move to support Matilda, except to fortify a series of castles across England - of which Devizes was one.
Eventually, their enemies (when did you last meet a popular taxman?) accused them of preparing to fund the campaign of Robert of Gloucester (Matilda's main supporter) - probably correctly.
Stephen contrived an excuse for rounding up many of Roger's supporters. His family was ordered to surrender their castles. When they refused, Stephen marched on Devizes, where Nigel of Ely (Roger's son) had retreated with his mother, Matilda of Ramsbury (Roger's mistress).
It was Matilda who surrendered Devizes Castle to Stephen, fearing for the lives of her lover and her son, under Stephen's threat to hang them both. After this, Roger's remaining strongholds were quickly handed over to Stephen (along with a nice little pile of money). Roger himself died towards the end of 1139.
As it happened, this act of brutality towards representatives of the church (however worldly) set the seal on Stephen's dynastic ambitions, as trouble flared up all over his precariously-held territories
In the years that followed, the fortunes of Stephen and (Empress) Matilda ebbed and flowed. Matilda was captured, Stephen was captured, Robert of Gloucester (Matilda's general) was captured (unfortunately, not all at the same time). Agreements were made; agreements were broken.
Meanwhile, England was going to hell in a hand-basket. Although mediæval warfare was a sporadic affair - with very occasional, small-scale battles - the removal of any system of authority, spiritual or temporal, meant that bandits could roam the countryside unopposed, and any petty local strong-man could treat his peasants like... well, like peasants.
One such, a Hervey Brito (Stephen's son-in-law), held Devizes Castle for a while, but so angered the local populace that they turfed him out, and handed the Castle back to Matilda (the Empress, not the mistress).
I do hope you're keeping track of all this.
"and as the two sides assailed each other they reduced all the surrounding country everywhere to a lamentable desert" - Gesta Stephani
The final straw for Stephen's dynastic ambitions was the death of his son Eustace, in 1153. It is a chilling thought that England might have had a King Eustace.
Eventually, war-weary (and probably as confused about who was winning as I am), Stephen and Matilda came to a compromise. It was agreed that Stephen would rule until his death, whereupon Matilda's son (Henry II) would succeed him - which came about in 1154. Matilda held Devizes Castle until her death in 1167.
Incidentally, Stephen was the last true Norman King of England. Henry II was Angevin (from Anjou), the son of Matilda's second marriage to Geoffrey of Anjou, and grandson of the gloriously named Fulk Nerra.