Q: What are eso npc guilds?

NPC (Non-player character) guilds in Elder Scrolls Online include Mages Guild, Fighters Guild, and the Undaunted. They are non-player run guilds which any character can (and should) join as early as they can.

Joining these guilds also gives you access to three distinct skill trees with different abilities: you can read more about it here.

Thieves guild and Dark Brotherhood (assassins) are confirmed NPC guilds which will be added in the near future, though no specific date has been set at this point.

Unlike NPC guilds, ESO also supports player guilds: every player in the game can start their own guild where up to 500 other players can join. Benefits are trading, easier grouping, and special guild chat channel.