Who was Sauron? (Possible spoilers for Rings of Power.)

One of the great things about The Lord of the Rings is that the actual Lord of the Ring has, like, two lines in the entire series. He's talked about quite a lot, but he literally only speaks that one time when Pippin grabs the Palantir.

He's even mentioned in The Hobbit as the necromancer. And for anyone who thinks that's a retcon, there are earlier sources where we find that the name of the necromancer was always Sauron, even prior to the LOTR being written.

[SPOILER ALERT FOR Rings of Power]

To anyone who has watched Rings of Power and read The Silmarillion, how accurate was that portrayal? He is actually portrayed as a rather nice guy, although as I write this I recall something about how Sauron was very charasmatic before some event that ruined his ability to transform and appear nice. Cannot recall what that was. Maybe the downfall of Morgoth or something?