HTML, CSS, JS

Would appreciate help with the result circle of this challenge. Couldn't figure out the fading effect on the bottom part of it, where it blended with the background.

I tried researching mix blend mode of CSS, but wasn't able to figure it out. If I was on the right track I would appreciate a nod on that direction and I can figure out the rest. If I was miles off, I would appreciate a bit more information on the subject.

Thanks a lot in advance for the time and help.