The College of Winterhold
Who should apply
Mages and/or pyromaniacs.
You can access the college from Winterhold, which can be found at the northeastern corner of Skyrim. You’ll eventually come across this location during the main storyline, but mage builds looking to level up quickly will want to make the journey early on in the game.
How to join
At the north end of Winterhold, there are some stone steps leading to the college. A Dunmer named Faralda will be there at first to greet you and block your path. Talk to her and tell her you want to join the college. She’ll then ask you to perform a magic spell to prove your worth. If at this point you don’t have any spells, she’ll sell you one for 30 gold. You can even convince her to accept you based on your shout. Once you’ve done what she asks, you’ll have access to the college and the Achievement/Trophy will pop.
Joining the college will give you access to full range of trainers in all the different types of magic. This is extremely beneficial once you have the gold to afford it. These trainers also sell spell books, either right off the bat or after certain faction-related quests. The main and side quests associated with the college will take you to areas where you can find some spell books and skill books, and the loot found is often mage-centric apparel or staffs.
By joining this faction you’ll earn the “Gatekeeper” Achievement/Trophy.
The reward for completing the main questline is the key to the Arch-Mage’s headquarters and the valuable Arch-Mage’s Robes. These robes increase your magicka by 50 points, reduce the cost of all spells by 15%, and boosts your magicka regeneration rate by 100%. This combined with the Morokei mask (which you can get during the Staff of Magnus quests and which also boosts your magicka regeneration rate by 100%) makes the rest of the game significantly easier for magic users. Finally, you’ll receive a permanent 10% discount when dealing with any vendors inside the college.
The Dark Brotherhood
Who should apply
Sneak builds and/or jerks who like murderin’.
The entrance to the Dark Brotherhood Sanctuary is a black door in the southwest corner of Skyrim, just west of Falkreath. Unless you’ve completed the steps below, however, you won’t be able to open it.
Unlike some of the other factions, joining the Dark Brotherhood won’t give you access to many trainers. You’ll only be able to level up your Light Armor with Nazir and your Alchemy with Babette. However, immediately upon joining you’ll be given the fantastic Shrouded Armor set:
Shrouded Armor: 32 defense, increases poison resistance by 50%
Shrouded Boots: 9 defense, silences your movements
Shrouded Cowl: 14 defense, 20% boost to bow damage
Shrouded Gloves: 9 defense, doubles backstab damage
Carrying out the assassinations quietly will naturally increase many of your skills, though it requires you to have some skill already. Also, the Dark Brotherhood quests are usually just plain fun.
By joining this faction you’ll earn the “With Friends Like These” Achievement/Trophy.
How to join
There are several ways to get started with the Dark Brotherhood using only information heard from NPCs. You can speak to any innkeeper or bartender and ask for rumors or gossip. You can find Maul in Riften and persuade/intimidate/bribe him for info. You can go directly to Windhelm (your goal) and look for Idesa Sadri on the streets. The objective is to hear about a young boy named Aventus Aretino.
Alternatively, you can circumvent all of that by simply heading directly to the Aretino residence in Windhelm. Pick the lock on the front door, enter the house, and find Aventus upstairs. He’ll give you the “Innocence Lost” quest. After you’ve completed it, find a bed to sleep in, and you’ll be awoken by a member of the Dark Brotherhood.
After you’ve completed her task, exit the shack and head towards the sanctuary, which will now be marked on the southwest corner of your map. Approach the Black Door found at that location and answer it’s riddle with “Silence, my brother.” Once inside, speak to Astrid. After doing so, an Achievement/Trophy should pop, signifying your entry into the Dark Brotherhood.
Note: If all else fails, the Dark Brotherhood will seek you out after you’ve killed enough innocent people.
At the end of the second to last main quest, you can obtain (legitimately) The Blade of Woe, a powerful dagger that absorbs 10 points of health with each hit. The damage output will scale up depending on the level and skill you are at when you obtain it. You’ll also net 20,000 gold for finishing the main questline, which is nothing to scoff at, especially early on in the game.
The Thieves Guild
Who should apply
Stealth builds and/or kleptomaniacs.
The Ragged Flagon tavern located under Riften, a city in the southeastern corner of Skyrim. Look for a gate underneath the central plaza of the city to find the entrance to the Ratway, which leads to the tavern.
Once you’ve joined the Thieves Guild, you’ll have access to trainers who can help you level up in most of the thiefy skills (pickpocket, lockpicking, sneak, and archery) in exchange for gold. You’ll also obtain several weapons and armors throughout the quests that buff thief skills like sneaking, lockpicking etc. If you have a stealth build, you should seek out the Thieves Guild as soon as possible to claim these rewards, which make the rest of the game much easier.
By joining this faction you’ll earn the “Taking Care of Business” Achievement/Trophy.
How to join
Go to Riften, located in the southeast corner of Skyrim, and look for Brynjolf. Talk to him and start the “A Chance Agreement” quest. Complete this little quest (make sure to save throughout, since it involves sneaking and pickpocketing) then head through the Ratway under the city to reach the Ragged Flagon. Talk to Byrnjolf inside the tavern and complete “Taking Care of Business” to join the guild and pop the Achievement/Trophy.
When you become Guild Master, Brynjolf will give you the Amulet of Articulation, which grants you instant success at any persuasion attempt and increases speechcraft depending on the level you are at when you receive it. Tonila will give you the Guild Master’s Armor set:
Guild Master’s Armor: 38 defense, +50 carry weight
Guild Master’s Boots: 11 defense, increases pickpocket success by 35%
Guild Master’s Gloves: 11 defense, improves lockpicking by 35%
Guild Master’s Hood: 16 armor, improves prices by 20%
You’ll also have access to three different Nightingale roles at the very end of the main questline, which you can change as you please after each day. Here are the roles, each of which grants a special power:
Agent of Subterfuge: Casts frenzy on any single enemy regardless of their level
Agent of Strife: Absorbs 100 health from an enemy
Agent of Shadow: You can turn invisible for two minutes
Finally, as guildmaster, all fences that you’ve discovered (NPCs that will buy stolen goods from you) increase their buying allowance from 1000 gold per day to 4000.