The Selunite Outpost is one of the many points of interest inside the Underdark. Baldur's Gate 3 players who are coming in from the upper northern tunnels are likely to see this small fort in the distance, assuming they don't accidentally find themselves inside it in the first place.
Baldur's Gate 3: How to Get Through The Selunite Outpost
This fortified outpost in the Underdark is just as hard to breach as it is to escape, but there are a few tricks that Baldur's Gate 3 players can use.