I looked up builds for NB tanks before eso released on console. Then when I started playing, I didn't like most of the suggestions. If you want to tank as a Nightblade, then consider my Blood Knight build. I will tell you which skills I use and why. If to you are looking into bring a NB tank then figuring out what passives to use is very easy.
1st bar: 1h and shield. If you want to tank you should be using a shield on at least 1 bar. Otherwise I'm not sure you should tank at all.
Abilities from left to right with --> morphs and why.
Strife-->Swallow Soul: As a tank you need all the healing you can get, especially for this build. Funnel health does not help you while solo and healing the group is the healers problem. If you want to heal the group, be a healer.
Drain power-->Sap essence: Damage trash, heal you and allies, and negates the damage loss from SA. NB bread and butter skill.
1h+'s Low Slash-->Heroic Slash: The general idea here is to constantly gain as much ultimate as possible. Besides, deep slash does nothing for you while fighting a solo boss.
Siphoning Strikes-->Siphoning Attacks: Nightblade bread and butter toggle ability. Must be on both bars so the ability remains uninterrupted. Regeneration procs with ALL attacks, which includes dots.
1h+s Puncture-->Pierce Armor: Ransack seems like a good idea since you are a tank. This one is debatable, but for this build you want to drop the spell resistance since you are a Blood Knight and when the enemy has lower resistance you are healing more. Plus, you have to be able to taunt.
Ultimate slot; Soul Shred-->Soul Tether: Only a healer would choose the alternate morph. This ability is truly awesome. Deals good damage, stuns, and actually drains health as opposed to just doing damage while healing. This skill will double as your trash pull and oh !&%
@$ ability.
The soul siphoner passive will bump up your healing ability by 9% in addition to the passive from swallow soul which increases healing received by 8%.
2nd bar: Dual Wield. DW for this bar for one ability only, but for several reasons.
Abilities, again left to right:
Telporting Stroke-->Ambush: Another bread and butter skill. This skill is your gap closer and is NOT affected by snare or immobilize. It also serves as a long distance interrupt on healers (pve), and increases the power of your next attack which is excellent to use before soul tether and swallow soul, or any other ability you want to augment and you have plenty of stamina to throw around.
Twin Slashes-->Blood Craze: The reason to dual wield on this build. Does small dd, big dot (procs SA), heals you over time (combined with your other hot's), and costs very little stamina. I considered a few other skills in place of this one, with entropy-->degeneration being a close second. The deciding factor was the higher damage and ridiculously low cost. Plus, the damage and heal can be augmented with sap and ambush.
Path of Darkness-->Refreshing path: I know what you're thinking, this skill sucks. But consider these facts; it does a small dot which is nearly negligible, but it procs SA. It heals you about twice as much over time as blood craze or swallow soul will given the same amount of time and you are on path. This skill also initiates shadow barrier, the passive that grants you major ward and resolve which at rank 2 and wearing full heavy, lasts 11 seconds. This replaces the need to use immovable from the heavy armor skill tree. This skill also mitigates speed lost from snares and is invaluable if you need to run from city guards. This build only has 2 abilities that use magicka and this is one of them which means you can use it often and it won't matter if you drain magicka. No other skill has so many uses, at least none I can think of that can be used.
Siphoning attacks again so the ability stays toggled on.
Bone Shield-->Spiked Bone Shield: Necessary for a tank. Big damage shield plus the damage done by the spikes while minimal also procs SA. The synergy also allows an easier "stack on the boss" mechanic for some fights.
Ultimate Slot; Consuming Darkness-->Veil of Blades: 30% damage reduction is sweet plus a large AoE dot that, you guessed it, procs SA. The synergy is awesome for melee dps that pull aggro also.
This build is mostly dots and hots as you can see. The point being that the dots will allow you to proc SA when you aren't able to attack due to cc or the need to block, and the hots will sustain you while soloing and assist the healer when their magicka is low. You will rarely need potions and as long as you learn how to tank, which you need to know no matter what your build, you will also be putting out more dps than most tasks which in turn makes fights shorter and taking strain off of the healer and dps. Anyone can taunt and turtle, but this build is about maximizing up time by mitigating damage with some self healing and high rate of resource regeneration. Each skill has multiple uses which are applicable to most, if not all, situations. Also, the constant resource regeneration allows for using these abilities quite often, though you must still worry about over spamming them.
I wear a full suit of heavy armor. No mix of light or medium. Many builds say you should use 2 pieces of light for 6% (at best) spell resistance. But armor is a one for one exchange of resistances meaning that the extra armor I get for going full heavy makes up for not utilizing some light armor. Wearing full heavy also increases the duration of your shadow barrier passive. Not to mention that creating armor with a higher armor rating will give you a bigger bonus If it is heavy. You can always make gear with the Nirnhoned trait should you want extra spell resistance.
Beginner level pull combo: Telporting Strike-->Lotus Fan, Soul Shred, Path of Darkness. Once you have soul tether, spend money to respec morphs and invest in ambush.
Lower level pull combo: Ambush, Soul Tether, (reposition yourself) Refreshing Path.
Higher level pull combo: Ambush, Sap Essence, Soul Tether, Refreshing Path.
Final trash pull combo: Tack on Spiked Bone Shield after path. The stun from soul tether lasts long enough to where you will have time to reposition and drop a path to catch your group of trash in the path and put up your shield. When they come out of it they are going to be pretty upset with you.
Boss fights: Other than dodging damage you will be cycling through pretty much all of your skills. Pierce Armor and Heroic Slash are priority, followed by sap and blood craze. Use ambush before soul tether and swallow soul for extra damage/healing. You may not be able to use ambush before each swallow soul, but be sure to use before soul tether to make it a 1-2 combo. Drop path when the boss is static and keep it up if and when they move. Also be sure to be on it yourself. In certain fights you will be constantly on the move so use your own judgment. Put up spiked bone shield as needed. Solo trash fights are roughly similar.
Good tank weapons are the Thalmor Gavel, which drops in Arx Corinium (I think) and Beastmaster's Crop which drops off of the Troll King in Blessed Crucible. A good offhand to use is Shard of Iceheart (Direfrost). I use Soul Siphon, but my friend gave it to me so i'm not sure where that drops.
Sorry for the long post but I hope you liked the read.
Keep in mind this is a PVE build.
Edit 1: Made some minor adjustments. Read below.
Update: With Imperial City patch, we kind of got screwed. The build stays the same for the most part except SA has been replaced by Grim focus on one bar. Read on for other input and some witty banter.