[this notice is pinned to the wall above the Tablet near the portal by a crossbow bolt.]
This is how the tablet reads:
"We, of the Illithyri people, have built this Way, filled with every delight, protected from every danger, to speedily transport from the island paradise above to our new realm in the quiet caverns below. May any not of our blood be destroyed by the Guardians"
The Illithyri were the elves that became the Drow. They fled the surface not out of choice but were banished to the Underdark for some great treachery. It might be an island up there they are bitter about leaving, it might be a lie and lead only to more of the Underdark, further up. Whatever it is, it is dangerous. Those Guardians will see through any disguise or charm and kill anything that's not a Drow.
Don't go poking around in there, folks. Have a plan. Lurue bless.
Egon "Balladeer" Rosenqvist. Bard, Luruan, and scholar of forgotten languages.