The Companions is substituted for the Fighters Guild, canonically aligning with Ysgramor and Atmoran lore prevalent long before Skyrim's inception. Thus, the College of Winterhold, making its debut in The Elder Scrolls V: Skyrim, appeared as a joinable faction. Due to the Mages Guild splitting into the Synod and the College of Whispers, the guild could not logically appear in titles set after the Great War.
While Skyrim kept the Thieves Guild and the Dark Brotherhood, the Fighters Guild and Mages Guild were exchanged for other similar organizations. The Elder Scrolls IV: Oblivion was the last The Elder Scrolls title to employ the conventional four factions, which were called 'Guilds.' These factions included the Thieves Guild, the Mages Guild, the Fighters Guild, and the Dark Brotherhood.
