Asmodeus, one of the most interesting characters in the series, will be referenced in Doomwyte. But before we find out how prominent his reference is, let's look at his past. Many Wikibeasts say he is a villain, but I say otherwise. I view him as neutral.
1) He indiscriminately ate whoever was in his path, even if it was one of Cluny's crew. He couldn't be good, or else no Redwaller would be eaten, and he isn't a baddie, as I'm sure he would eat Cluny the Scourge and King Bull Sparra if he had the chance.
2) He kept treasure that he won through defeating his enemies. This behavior is comparable to medieval dragons in fiction, as they kept treasure in a similar manner. Medieval dragons were typically neutral (occasionally good or evil) and often had reason to destroy people who were taking his/her property. Keep in mind Asmodeus didn't destroy Redwall, but otherwise he behaved like a medieval dragon.
3) He was only killed because he was an obstacle in the path to success. Martin the Warrior knew Matthias needed to save Redwall Abbey, so he had the snake offed. If the sword wasn't desperately needed to kill Cluny, Asmodeus would still be alive. Additionally, Matthias' own life was at stake because he intruded on Asmodeus' property. If Matthias died, so did Redwall.
And... sadly, my argument has flaws. Shall I argue against some of them?
1) Asmodeus' name: His name derives from a devil. Is that enough to make him a baddie though? You argue that Veil's name is an anagram of "evil" and "vile", yet the story is about him choosing between good and evil.
2) If Asmodeus were neutral, can we consider Skrabblag and other creatures neutral via this information? No, because Asmodeus doesn't belong to a particular faction. He is an individual, much like we Wikibeasts are.
3) He is indirectly responsible for the events in Mattimeo. True, but who is the antagonist in that book? Slagar or Asmodeus?
4) In Redwall, Asmodeus is described as being evil through numerous adjectives. Hmm... haven't thought on this one. The descriptions seem vivid enough. Too vivid, perhaps? Almost as if Kilconey were exaggerating... (Flaw from Essay:Asmodeus' Neutrality:A Rebuttal)