Ok random theories that my brain came up with regarding Feliar and Vicoria relationship:
1. They were married once, or at least courting, they know one another way too much for people who dislike each other that's kind suspicious, on one of Feliar's dialogue options he says something like "... I love shittalking about my..."(not his exact words) and never finishes the phrase, sentence(?), Leading one to believe they have/had a closer relationship before coming to despise each others guts, also Feliar doesn't mind the use of aphrodisiacs and potions with similar effects on him and he also can keep lucid enough under the effects of the Love potion to the point of acting pretty normal just really horny, which is very suspicious since the only other person who somewhat keeps control is Xer and that is probably because he has some resistance to potions and even he doesn't seem to remember clearly or at all what he does under the effects of the potion, which is not the case with Feliar since he remembers crystal clear what happens and what he does under the influence of it, so that leads me to believe knowing that Vicoria loves her aphrodisiacs that Feliar built some immunity or a very strong resistance to those during their marriage, but due to some reason their relationship broke apart.
2. Feliar is actually Vicoria's son 😱, this one is basing itself on the same dialogue option I mentioned above and is only for kicks, because there's no way in the seven hells that they're related that way, but it's fun to consider this possibility.
Those are the only theories on the nature of their relationship I have for now maybe I'll get something more when we get a plot advancement on that part of the story. Now a theory on who left behind that message about Feliar underground:
It was a friend of Feliar who bought that the larvae are good for sexual durability but unfortunately was very human and ended up becoming a tar being, or, one of his subjects or a lover that he had made a promise with but he didn't keep his side of said promise. Oh yeah btw I think that Feliar is king of the satyrs for no apparent reason.
Random thoughts on what's behind Feliar's door:
. A portal to the underworld
. A room filled with riches and treasures
. His personal quarters(if it's just this I'm gonna be sad because of all the mystery of the door throughout Feliar's encounters)
. An underground series of tunnels that leads to the Satyrs's village
. A prison filled with nightmarish creatures
. A storeroom(boring option)
. Secret tomes about the nature and history of the Nameless and his goals as well as the witch of the swamp
. Another gem