Tibia is an old game. Very old. Old enough that the developers could add a more-or-less throwaway joke a decade ago, a door that could only be opened by someone at level 999. At the time, this was more or less impossible, as no one was even close to level 999. Years later, it’s still an insanely high level… but now, someone is close to level 999. Just one level off, in fact.
The character named Kharsek is now level 998 and well on track to reach 999 in a few days, thus allowing him to be the first to pass through the door and see what’s there. Whether or not he’ll reveal what’s behind the door is a subject of some debate in the community; he’s under no obligation to do so, but many people are anxiously hoping to find out what lies beyond a decade of mystery. There’s no way of being certain that it’s worth the buildup, though.