Your time on the Nautiloid in the prologue is simply a plot device. You don't actually adventure or fight in the Underdark in the prologue. You fight aboard a Nautiloid flying through through one of the levels of "hell". Combat is minimal, against low level opponents, while the high level Illithids are busy dealing with the demonic hordes. This sets up both the Ilithids and the demonic leadership as future protagonists. Entry to the Underdark occurs later in the game. It takes place in what are essentially the top levels in bits of the Underdark, barely subsurface, and unless you are playing "A Faerun Day at the Races" hurrying to get there, you will likely be level 3 or perhaps even 4 when you first enter it.
Annakin Skywalker is way out of his league when caught between the influence of the Jedi and Sith - that created a story.
Picard was way out of his league when put on trial defending humanity before the Q Continuum - but it created plot lines for years.
One inexperienced hobbit was totally outmatched with everything he faced in Lord of the Rings, yet he grew, matured and overcame.
Another one, in an earlier time, failed and was seduced by the ring.
High stakes prologues that are beyond the initial comprehension of the protagonist are an old baseline in fiction.
