Honestly, it's really hard to come up with an answer to this. Just like the 469 Language, there has been multiple "answers" to this mystery but none of them has really shown any effective result. My view on this thing is that it's probably an unfinished quest, or mostly added for lore purposes. And here's what I think about it:
1. The Pits of Inferno (POI)
It's no mystery to anyone that the POI was one of the big mysteries of the game back then, when it wasn't yet quite finished.
POI Entrance: https://imgur.com/a/o1Ovv2U
To quote some user from the tibian reddit:
"Doesn't even need Cip to explicitly say it, they have been doing those things for years. Once upon a time POI was one of the biggest mysteries of the game. They teleported Arieswar to the entrance during testserver to tease people. People ran around PoH like crazy, picking every single pick-tile. Couple years later they introduce the quest, spoils it all over their website and all old school mysteriando died over night."
Source: https://www.reddit.com/r/TibiaMMO/comments/1ebn2em/tibia_mystery_iceberg/
That way we can at least consider the fact that it isn't unprecedented the fact that cipsoft really actually leaves unfinished quests around, until later when they actually finish it. Considering the fact that the path to the Pits of Inferno actually existed before it was finished, leaving no way to go from there.
2. The Leaked Files
I think the most common source of information over the unfinished quests are the infamous leaked files from the 7.7 version of the game. It isn't a "clean" source per se, but it does bring a few interesting points.
2.1 The lack of exits to anywhere from the cave, above or below:
The cave where the Basilisk Stands: https://imgur.com/a/Kc4bfZ5
The floor above it: https://imgur.com/a/OL5bJVK
Even though the cave itself shows many yellow dots as "caves" there's no way to go from there, be it above the cave or underneath it.
2.2 What the files show:
Some content creators that rendered the files into an actual unnoficial server show what was actually in there before the 7.7: https://imgur.com/a/o8iVob2
It's interesting to remind the reader that the Basilisk was already a mystery back then, just as today.
The cave that would theorically be the path where the Basilisk fought the Dwarven Soldiers off, before going into his retreat, doesn't actually lead anywhere: https://imgur.com/a/QaltYCO
The exact spot, with the coordinates on the map (6th and 7th floor): https://imgur.com/a/0RCr5iM
2.3 The Basilisk is an NPC
The fact that the Basilisk is an NPC and not a creature is probably a widely known fact. But what does the NPC say? According to the source... nothing. There seems to be no quests or rewards related to the NPC itself or the cave around the Basilisk.
The Basilisk codes as NPC: https://imgur.com/a/Fx0QZ4t
So, in conclusion: Yes, players have accessed the room through unnoficial ways and what they found out was not exactly what people expected anyway. What can we get from this is that either the Basilisk was an unfinished quest up until the 7.7 version of Tibia, or it's a mere piece of the rich lore from the game.
I can't say that it's a dead mystery, just like any other, because there will always be people researching about it. Who knows? Maybe after all this time they actually added something to it. But basically, what we have so far is what I listed above.
Sources:
https://www.youtube.com/watch?v=z9qMjUh4jV0
https://tibiamaps.io/map#32640,31915,14:3
https://tibia.fandom.com/pt/wiki/Basilisk
https://tibia.fandom.com/wiki/The_Pits_of_Inferno_Quest
