This season is so much better then season 1

I have a question: how was Sauron able to take an elvish/maia form? When he first emerged he was a black fluid, and only after killing a human he could take a human form. I can only imagine him killing an elf and then implying he's more than that by showing some of his true powers to Celeborn... But did that really happen? What am I missing? PLEASE answer from the TV show only. As an adaptation, it should be consistent on its own, so I only care for answers that come from what we've seen so far