Um, what? All you know, and all anyone knows, at the beginning of Oblivion is that the Emperor got axed. The Emperor, voice acted by Patrick Stewart, whom you met and talked to as to give weight and meaning to the world shattering event that by itself would have meant nothing to any new player. You learn about Mehrunes Dagon way later on, and the game eases you into that too. You're not just dropped into some random inn, given a quest and asked to read on some random book about some God you know nothing about, because that would suck.
Except that that's not exactly true, right? We not only know the King got whacked, we see who did it, and they're trying to whack us too. Subtext: We are drawn into a world shattering event, from people that we know nothing about. Nowhere did Oblivion pause the game and explain who the Mythic Dawn worships. I may be misremembering the name of the cult. What I am not misremembering is that they were worshippers of Mehrunes Dagon, and, as you point out, a new player would have no idea what that means, or who it is, and we don't get that information until much later.
So if a new player starting in Oblivion had gone to the forums complaining that they didn't understand why this cult they know nothing about was trying to kill them, what would the response be? I'm going to go out on a limb here and say it would be a lot different than what we're getting here, right?
The trick here is that you as the player aren't *supposed* to know who they are at that point, and we don't need to know either. Like you said, we do get that information, just later on. I can't completely remember the context of when you learn about him, but I don't think that learning specifically who they worship is meant to be a big twist. If I recall correctly, it doesn't strictly matter who exactly they worship. All the player needs to know is that he's a world-ending threat. All the other detail about who he is in the context of the world we exist in. The why of it doesn't really matter because, knowing he wants to destroy the world is enough of a reason to oppose him. But in the case of BG3, it really does matter that we know who Shar is in the context of the world because otherwise we don't have any reason to care about what Shadowheart, our companion and a major secondary character, is telling us. If the new Oblivion player went to the forums, the answer to their complaint would be "wait, you'll get that info later." Maybe they could read books and figure it out earlier based on hints and context, but that's not going to fundamentally change their experience of the story. In this case however, the difference between an emotionally sattisfying moment and a confusing one is whether or not you read and noted the importance of a couple random books that might well have just been flavor, like most other books in the game are.
