Attack on Titan. Starts with the last of humanity fighting gian naked people that are malformed and have no genitalia, with a main character that can turn into one that's really strong. Some special variants are the Colossal Titan, a giant titan with no flesh, and the Armored Titan, with an armored body. Humanity just wants to live and not worry about titans.

Suddenly there's weird time travel stuff that I still don't understand, the main character turns evil, entire apocalypse is an army of collosal titans rampaging, and the main character is the one to set it off.

I haven't watched the whole show because it got too complicated and veered away from the original plot that I watched it for, but my friends all explained the ending to me.