

Shivering Isles rivals Morrowind in my mind. It has a strange and unique setting and most of the content is incredibly well-written, which contrasts sharply with the standard medieval setting of baseline Oblivion (mandatory reminder that Cyrodiil was supposed to be a rainforest, but the devs retconned it to make development easier).
The other expansion, Knights of the Nine, was just a bunch of fetch quests to unlock an armor set and was disappointing in comparison to even the base game (though at least the final boss fight was cool). It also put behavioral tracking on the DLC’s rewards that would disable them if your character gained infamy, forcing you to repeat a bunch of boring travel quests to fix them whenever this happened. There’s a reason KotN never comes up in discussions about the game.
(I know this is a shitpost, but I couldn’t resist lore-dumping)
The Dark Sign is actually Gwyn’s curse on humanity to seal away their potential (the Dark Soul being the only fragment of the First Flame that can be shared and passed on without weakening, he feared humanity growing powerful enough to topple the gods through sheer numbers). That’s why the Dark Sign appears as a flame encircling the Dark. When the First Flame weakens enough, his seal becomes visible as the Dark within humanity begins breaking free.