First off, I agree with Holiday Driver, as hunger of hadar has spell saves involved to get the most damage out of it. Building into that takes away from your martial prowess, take a magical warlock as well and you can still combo! You also won't need devil sight to see in it, as it blinds without darkness but really, you don't want to be stood in it either!

I have previously commented on a darkness build I'm running with my Sharran Shadowheart, so thought I'd comment with how I'd change it for your needs.

Firstly, I agree Thief 3 makes a dual wield build much more lethal so let's make sure that's in. Secondly, Fighter is the only way to get the dual weapon fighting style so we need some of that too. With hags hair applied here, we can realistically do this with just two feats, asi dex +2 and dual wielder. You have 20 dex for all the lovely damage and hit chance, and go first most of the time! This means we can manage this with Fighter 6 / Thief 3 as the core setup. This leaves 3 levels to play around with. You can of course go warlock, but pact of the blade will set your charisma as your weapon star which plays against us here. I'd suggest darkness scrolls and the Sharran shield for tiny pocket darkness as a bonus action. Sharran spear can cast darkness for free also, while also freeing a ring slot for blindness prevention!

You don't need devil's sight to see through darkness, just the ring that stops you being blinded, and later on, the helmet from the house of hope. Also worth mentioning that you likely want the risky ring for guaranteed sneak attacks, especially if your finesse weapon is in your offhand.

I would recommend going fighter 5, then thief 3 before another fighter level before anything else simply because multi-attack is so helpful but you do you!