[Spoilers] What now?

[Cactx]

I just finished going to the blacksmith to repair my nail, and explored The City of Tears, the Waterways, and the Soul place. Do I fight the Mantis Lords, or that dark knight on my map at the Forgotten Crossroads?

[FerroBorne]

Fight The Mantis Lords can give you the access to The Deepnest which is give you access to kill Herrah The Beast. The Dark Knight you said in Forgotten Crossroads is the Final Boss - Hollow Knight, In order to open the Black Egg to fight him you'll need to kill 3 Dreamers.

Monomon the Teacher in Fog Canyon after defeating Uumuu boss

Lurien the Watcher after defeating Watcher Knight in "Watcher's Sprite near King Station/ other side of City of Tears

And Herrah the Beast in the farther left side of the Deepnest

[Dan]

If you explored the Waterways, then you should have beaten the Dung Defender; you can get an item that lets you enter the acidic water after beating him, but iirc it requires the Crystal Dash. You can get that from the Crystal Peaks. And definitely get the Dream Nail from the Resting Grounds if you haven't already.

[Wondercrab]

You're at the point where the game starts to branch out a lot, but your next destination should probably be Crystal Peak (accessible via the northeastern area in Forgotten Crossroads with the pink crystals), then down to the Resting Grounds after (there's a big drop down a chasm in Crystal Peak that'll take you there). From that point you'll gain access to an important item and your next set of major objectives will become accessible.

[Cactx]

i already accidentally stumbled down there to the crystal peaks early in the game (i didn't waste 18,000 for a Lumafly for nothing) and i already have the dream nail and 95 essence. thanks everyone who participated :D