Joining an NPC guild in The Elder Scrolls Online can be a great way to gain access to new content, rewards, and powerful abilities. The Fighter's Guild, Mage's Guild, and Undaunted are all popular options to consider. Before joining any of these guilds, it's important to understand the pros and cons of each as well as the requirements for joining. 

• Fighters Guild
• Mages Guild
• Undaunted Mettle
• Citric Order
• Dark Brotherhood
• Thieves

Fighters Guild

The Fighters Guild is a great choice for those looking to get involved in PvE content. The Trap Beast ability makes fighting enemies much easier, while Intimidation allows players to skip dialogue options in certain quest lines. Slayer III is great for those looking to boost their damage against monsters, and Banish the Wicked is useful for generating Ultimate when killing enemies. Dawnbreaker is a powerful Ultimate for those looking to get involved in PvP, and Bounty Hunter allows players to accept bounties in Cyrodiil. All these abilities are easy to level up as progress is gained passively by killing undead creatures in the world. 

Mages Guild

The Mages Guild is a great choice for those looking to get involved with magic-based gameplay. Entropy and its morphs are great for PvE content, while Passive Will allows players to skip dialogue options in certain quest lines. Magicka Controller and Might of the Guild are both powerful abilities for PvE gameplay, and Everlasting Magic allows players to heal and buff allies. Inner Light is a useful passive ability that grants players Major Savagery, Prophecy, and a 5% Max Magicka buff.

Undaunted Mettle

The Undaunted is a great choice for those looking to get involved in PvE content. Players are invited to join at level 45, and they are rewarded with a free key just for showing up. Those looking to tank can benefit from Undaunted Mettle, while those looking to increase their stats will benefit from Undaunted Command. The Undaunted can be leveled up by completing pledges, dungeon quests, and veteran dungeons on hard mode. Completing these gives players double keys as well as extra Undaunted experience. 

Citric Order

The Citric Order is a guild found in the Somerset Storyline, which is available in the Elite and Deluxe versions of ESO. To join the Citric Order, you simply need to progress through the story and reach the second quest. This guild is one of the easier ones to join, but it does require some dedication to level up. As a member of the Citric Order, you will gain access to powerful abilities such as Minor Protection, Channeled Acceleration, and Men's Wounds. You will also gain access to the Elly Weapon, which is a powerful weapon with great utility in both PvE and PvP. The Citric Order also offers the Passive See The Unseen which allows you to loot special portals and gain access to some great items.

Dark Brotherhood

The Dark Brotherhood is a popular guild in ESO and is often rated as people's favorite NPC guild. To join the Dark Brotherhood, you need to be in the Gold Coast, where you will find the first quest of the Dark Brotherhood storyline. As a member of the Dark Brotherhood, you will gain access to the Blade of Woe, which is a powerful weapon that allows you to assassinate targets and steal from them. You will also gain access to the Pitless Justice skill, which reduces your heat and grants you Major Expedition after you kill someone with the Blade of Woe. Additionally, you will gain access to Shadowy Supplier, Shadow Rider, and Deliberation, which are all great abilities for different purposes.

Thieves

Finally, we have the Thieves Guild, which is found in the Hughes Bane expansion. To join the Thieves Guild, you need to progress through the story and reach the first quest. As a member of the Thieves Guild, you will gain access to abilities such as Haggling, Timely Escape, and Veil of Shadows. You will also gain access to the Heist Board, which offers daily quests for great rewards, including set items, motives, and more. Additionally, the Thieves Guild offers some great titles and furnishing items, making it an excellent choice for those who enjoy a bit of flavor in their game.

No matter which guild you choose, you're sure to have a great experience in ESO. With each guild offering unique rewards and experiences, you'll be sure to find one that fits your playstyle. So make sure to take your time, enjoy the journey, and reap the rewards of joining one of these great guilds.