I've explored every part of the Abyss.....all that is left is a blocked door with a sorta wing symbol on it that I can't pass
The solution is in the Veil. I'll give you the details below if you don't want to keep looking on your own.
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in the west part of the veil (that is, the part you enter from open waters), you can find a diver swimming around. He's sort of right of the center, in the open parts of the water so he's kind of easy to miss but if you look for him you'll find him. Chase him around and he'll flee into a cave. Keep chasing him until he's cornered, then use bind to steal his helmet.
After this, you can go to the bottom-left part of the abyss, to the big locked door, and open it by swimming up to it and right-clicking on it.