Well let's look at what we know so far. Spoilers for descent into Avernus below
The Prequel to the game has a corrupt city with gang wars, a refugee crisis and three evil Gods of Murder, Tyranny, and Undeath, and their servants running around, the Undercellar a massive bar/brothel/black market/entrance to the Underdark, public fiend cults, a nearby city that gets pulled into Hell, betrayals, the Bloodwar, Demon Lords, Tiamat Hell Goddess of Chromatic Dragons, Archdevils, the option to trade your soul to spare your life, Infernal Machines that run on destroyed souls, undead, warlords.
And that was just the prequel. We know the Mindflayers which eat brains, conduct horrifying experiments on folks, and turn people into more of them are in and they have a giant Spelljamming Nautiliod.
The main character and possibly the whole party has something dark inside of them, possibly a mindflayer tadpole that is making them stronger instead of turning them into a Mindflayer.
We know there is a three way genocidal war between the the Illithids, Githyanki (including ones riding Red Dragons), and Githzerai and it appears to have spilled over into Faerun.
So yeah its sounds pretty fucking dark to me.
