Yeah, Silenus is one of the “middle” 50% mana starters (along with Holy St. Astra and King Avelorn) that are a bit tricky to place.
I put him in C because a) I don’t think you can build around his spell - 104 damage is pretty good, but requires a full wildfolk team, and +3 magic and burn-all are ok side effects, but not game changers. 14 mana cost is faster than average, but overall, his spell seems like a lot of ok pieces that don’t quite add up to something great. Compared to higher-ranked spell casters like King Highforge (similar damage, but inflicts stun to two targets), King Avelorn (more damage+summon), or Krystenax (more damage, board control+summon), I think King Silenus falls a little short.
This leads to b) in terms of adding value to a team, Silenus works well with The Wild Queen, but The Maraji Queen also gives her a fast start, with much better team options than King Silenus (elementals overall >> wildfolk). I’d still peg Silenus as a B-tier, if there were other good wildfolk options he could boost, but there really aren’t any other strong wildfolk spell casters who benefit from his fast start (Luna’s good more because of her third trait, rather than her spell).
All that said, Silenus is still pretty good, and if there was an all-wildfolk team without TWQ (or a TWQ that was as good as her elemental teams), I could see him moving back up to B pretty easily.
Also, the new kingdom bonuses tied to deeds may also move King Silenus up the ranks, something I hadn’t really considered when making the last version of the tier list. (The stat boosts of an all-Pan’s Vale team may be enough to make him a favored partner of TWQ again.)