When Slagar/Chickenhound tells his story to Mattimeo, he says that he was kept 'in what the jokingly called, an Infirmary' but we never actually saw him being held in the infirmary in the TV series.
Slagar: You want to know the truth mouse?! Matthias, warrior of Redwall, will know pain when his only son is meant to suffer the way Slagar suffered, and that's a promise!
Slagar: (takes off his mask) Your father did this too me.
Mattimeo: No I Know the truth you did it too yourself!
The part about being consumed by Asmodeus's poison was completely left out of Slagar's/Chickenhound's story.
Slagar's true identity becomes known for the first time.