There are spoilers here for The Elder Scrolls IV: Oblivion’s Dark Brotherhood faction here.
The first half of the Dark Brotherhood questline in TESIV, up to an including ‘The Purification’, and the player’s arc through that organisation is Oblivion’s strongest content. While most of its appeal is certainly down to its quest design (which is, simply, better and far more imaginative than the rote collection or killing quests seen elsewhere), it also distinguishes itself from the rest of the game by thoughtfully constructing a community within the Dark Brotherhood. In a game also featuring a Mages, Fighters and Thieves Guild – not to mention a lengthy Arena career – the Dark Brotherhood, alone, manages to make the player feel a member of an organisation rather than its sole saviour. Other guilds are acceptable vendors of quests, but the Dark Brotherhood exceeds them in one important respect: only there does the player become attached to the group’s NPC members, seek them out for advice and relish interactions in the Sanctuary, the Brotherhood’s headquarters. This is all the more impressive since the Brotherhood stands for the values most alien to most players, worship of an ancient, evil deity and taking pleasure and pride in murder. There is, of course, a dramatic shift in the quest line halfway through, after which it becomes much more of Oblivion’s standard fare, but is first half stands so far above the rest of the game’s content it’s worth examining why, and particularly how the player comes to feel they have joined, in a real sense, a ‘brotherhood.’