I have to say that Satan's appearance in LoS was a complete shock to me. When he first appeared (I think that he had somehow possessed Zobek, then used him as a sort of gateway, burning through his body to come onto the mortal plane) I thought he was Dracula. And I was like "(o_0)WTF!?, that's a really weird way to finally throw Dracula into the mix. Just have him suddenly show up at the end. What's the point other than it's Castlevania, so Drac's gotta be the last boss?" because up until this moment I still thought Drac would be the end boss.
But when he looked up and said "FATHER, I come for you! Now YOU will bow down to ME!!", right after that last line, I literally said out loud "Oh...my...GOD! It's Satan!"
Even during the old school games, I always figured at no matter how powerful and evil Drac was, considering that Castlevania's has Christianity as a background wordlview meant that satan was ultimately behind it all. So to me, having satan as the main villain for Castlevania makes perfect sense.
Would still like a return to form after a few games though.