The article below is just a basic introduction to the topic. For something more advanced, written by pros and a fantastic leveling guide, my recommendation would be this Elder Scrolls Online Leveling Guide. I could not find any other guide that comes even close in terms of the impact it had on me.
Like all of the four classes in Elder Scrolls Online, Nightblades will be able to choose from four faction-based guilds. These four guilds are the Mages Guild, the Fighters Guild, the Werewolves Guild and the Vampires Guild. More guilds will be added to the game in later updates.
Players will be able to join multiple guilds at once. However, since each guild grants players with unique skill lines with both active and passive abilities to choose from, it may be best for Nightblades to focus on one or two guilds in order to have the best skills available. For the Nightblade, this will generally mean the Fighters Guild, but the Mages Guild has some solid options as well.
The Fighters Guild is ideal for classes who fight in melee range as well as classes who plan on fighting a great deal of Undead and Daedric enemies. The active abilities and the ultimate ability in the Fighters Guild skill line all focus on dealing damage to Undead and Daedric enemies as well as dealing AoE damage and providing group utility. The passive abilities are also aimed toward Undead and Daedric enemies.
In addition to new skill lines, players will gain access to faction vendor items and unique storylines by joining the Fighters Guild. The vendor items may especially be worthwhile to Nightblades, depending on the types of items available.
Other Guild Options
While the idea of a stealthy, blood-thirsty Nightblade joining a guild for casters seems a little strange, the Mages Guild may be a good fit for many Nightblades who use a great deal of Magicka. The skill line has some decent active and passive abilities. The active abilities feature two that deal damage over time as well as leech health.
One of the most useful passive abilities is one that increases maximum Magicka as well as Magicka regeneration for every Mages Guild ability slotted. A decent Nightblade soloing build might consist of a few actives from this line in order to take advantage of this passive. This would allow the Nightblade to spam abilities without having to worry about running out of Magicka.
Both the Vampires Guild and Werewolves Guild will likely have some solid skill options for Nightblades as well. It will be important to check both the skill lines as well as any possible reward vendors to know which guilds are worth pursuing. Upon joining a guild, a player gains immediate access to the skill line which makes it easy to decide.
The article above is just a basic introduction to the topic. For something more advanced, written by pros and a fantastic leveling guide, my recommendation would be this Elder Scrolls Online Leveling Guide. I could not find any other guide that comes even close in terms of the impact it had on me.