I don't think it is that early
That really depends on your gameplay tho ...
I also rarely ends up inside Underdark, while i still have business on the Surface, but POTENTIALY ...
If you just run there you can:
Lets say you start from the Groove ... bcs you were send for Halsin ...
You either follow priestess Gut to her lair, or you trespass there ... kill single ogre, easy peasy ...
You can solve a quite easy puzzle, or you simply randomly find the lock that bypass it ...
You are in the Underdark now ...
You can notice that one window is broken and leave there, other way around than petrified Drows ...
Then all you need to do is simply go left ... and voila! You are there.

Now all you need is to either pass fairly easy Str or History check, wich you can try with each party member separately.
See? Its not that far.
Sure, you can go around ... but that is different story.
You can also fly from Berlin to Paris, over New York or Moscow ... but that dont make it travel for several days, you just picked ineffective route.
Contrast that to getting heat metal as my bard and burning the devils hands right at the beginning of the game to get everburn, an absurdly powerful starting greatsword for doing almost nothing except for maybe save scuming to get the role I need for heat metal to land.
Yeah I am not following the logic.
Quite honestly? Me neither.
All Everburning Sword does is that is saves single Bonus Action to dip ... once per 4 rounds.
How is that absurdly powerfull?
I have no idea.

Especialy if you put on other side of scales facts that you cant hide in shadows while holding it, since it counts as source of light (ok, that can also be benefit) and that its blade on the contrary to regular Iron, non-magical Greatsword cant be poisoned, when you are killing Fire-Resistant enemies (like Tieflings).
And in order to obtain this item, you need to kill an enemy that is totally able to one-shot each and every member of your group, and have more than double amount of HP than your WHOLE party counted together.

There is also time limit of 17 (or 14?) turns for that combat in final release, if you dont kill him in time > game over.
I say this loot is well deserved.

I also don't expect it to be Excalibur powerful, a 1d4 radiant damage or a light enchant would be more than enough.
Sounds reasonable ...
You know what i would like? Mash it together!
1d4 radiant damage, when Light enchant is active ...
I wouldnt even mind if it would provide Light Spell.
Yes i know people around here are alergic to conditional effects, but im addicted to them.
