The achievements in Oblivion, and consequently Oblivion Remastered, aren't the most difficult to achieve. This is evidenced by the fact that, in the first 24 hours since the remaster shadow-dropped, several players earned every achievement in the game.
The majority of Oblivion's achievements are related to achieving certain ranks in the game's various factions, which just translates to completing quests in each respective quest line. There are still rare achievements, however. Not because they are difficult to achieve, but rather because not many players have completed a certain quest line yet.
If one were to look at Steam's global achievement statistics for Oblivion Remastered, one would observe that the seven rarest achievements in the game are all for achieving ranks in the Court of Madness, the faction that underpins the Shivering Isles expansion.
The two rarest achievements are both related to the same choice during the quest line, choosing whether to become the Duke of Mania/Dementia. You can't get both achievements during the same playthrough, but you can always reload a save to get both.
Since these achievements are mutually exclusive, they're actually rarer than the achievement for finishing the entire quest line. Duke of Dementia has been achieved...
