Who exactly even is “the dark one”?

So on my first play through which I just finished up, I was a warlock in a pact with the fiend class as a drow half elf. I knew nothing about dnd and kinda just picked whatever class sounded most appealing to me. It was also a mostly good run (I killed some people if they made me mad like wulbren lol)

Since I knew nothing about dnd I thought it was just kind of like being employed or something irrelevant to a boss but it seems more complicated than that, and I’m curious who exactly I patroned to especially learning more about wyll.

I’m still very new so forgive me if I got some info wrong!!