spaaaaaaace

Pros + An unusual but worthwhile build + Keeps most of Shen's tankiness while adding assassination abilities + Stand United is more useful than ever + AoE taunt is still kickin' yordle butt + Energy-based skills means no need to conserve mana + Perhaps one of the most efficient assassins actually (explained later on)

space

space

Cons - A missed Taunt hurts a lot in a trade - Awfully long waiting times in Teambuilder, people quit or flame in Blind Pick - Obviously loses some of the usual Shen's tankiness - Energy-based skills means you can only use so many of them in a small time frame - You need good reflexes and knowledge of your enemies to survive and farm enough early on

Ki Strike: One of the many ways to output damage as Shen, we'll be capitalising on using our passive efficiently, as only Shen's abilities aren't enough to take down a fully healthy enemy. With some attack speed, given by Nashor's Tooth and possibly Wit's End, Ki Strike will be proccing all the time, and with the extra health from Rylai's Crystal Scepter and possibly Liandry's Torment or Randuin's Omen it will hit a lot harder too.

---

Vorpal Blade: Shen's Q deals surprising damage early on and has a nice 0.6 AP multiplier. Sneak one of these in whenever your enemy tries to farm, and you'll have them down to their last health points before long. However, always take care to not be harassed in return ( Feint will help with this) and, if need be, use Vorpal Blade to farm instead of harassing.
Apart from damage, Shen's Q provides sustain. Autoattack a minion after hitting it with Vorpal Blade to regain some health over time, or execute it with Vorpal Blade to get a smaller amount of health instantly.

---

Feint: This ability and its 0.6 AP ratio means that by building a bit of AP instead of health and armor, Shen won't lose much of his tankiness, as he'll have a ~500 health shield available almost always at late game. Don't be afraid to use Feint, as your energy refills too fast to need to worry about it. Use it to block or bait harass, and in every other case you'd be in danger.

---

Shadow Dash: While it is never used for its damage in a usual tanky Shen, his taunt does deal a considerable amount of damage and helps out your combo a lot. Be careful though, since using it to attack means not only forfeiting half your energy, but also being in the mercy of junglers or the enemy midlaner themselves should you fail to land it. In this case, instantly Feint and back away.
Moreover, remember how assassins like LeBlanc and Talon recently lost their precious silence? A taunt includes a hefty silence as well, rendering the enemy unable to cast any abilities for the duration. Take that, mainstream assassins!

---

Stand United: Shen's famous (and notorious) ultimate ability has a whooping 1.35 AP ratio, which means that the shield's health benefits from 135% of your AP. This leads to some stunning 1500+ health shields around endgame. It also gets you in the thick of a fight, allowing you to instantly Taunt multiple targets, take down a squishy, soak some damage and save a teammate at once.

As for runes, I focus on AP along with some magic penetration and armor. Armor runes in mid are always useful, as they help both against AD assassins and the autoattacks of enemy mages. The rest are pretty standard, nothing extraordinary.

---

My masteries are also pretty self explanatory. A little bit of tankiness and regeneration, coupled with CDR, ability power, and Spell Weaving and Blade Weaving , which can be invaluable during a combo.

---

As for summoner spells, Flash is obviously a given. I generally prefer Teleport over anything else, as map presence is invaluable with Shen, at least for me.
Other viable choices would be Ignite to help with your damage output in that killing blow or Exhaust against AD assassins.

I wanted to add the Items section before this one, but I realised that you need to understand why you're told to buy these particular items. Here goes.

You've stepped in the fields of justice, gripping your ninjatōs.
You take a step, your armor producing a telltale sound of leather rubbing on leather.
As you notice it, you will yourself to be more careful. Your next step is silent.
You look up to the sky, smelling the fresh air, calm and dreadful.
Another fight begins.

---

EARLY GAME:
The very early game is when Shen is at his most vulnerable. And by very early, I mean no later than level 2.
Farming at level 1 can be a nightmare against a good harasser. You should have your Vorpal Blade. Use it to last-hit minions. Try to execute them with Ki Strike as well. To be honest, very few times have I made it past level 1 with more than half my health.
The moment you reach level 2, however, you're way safer. With your shield and the regeneration from your Vorpal Blade, you should keep your focus on farming and blocking any harassing attempts, as well as regaining your health with your Q and autoattacks. At around level 3 you should have almost full health again.
At level 3, assess your situation. If the enemy jungler is dangerous and likely to gank you, drop a point in Shadow Dash just to be safe. However, you should only use it to attack if you're ABSOLUTELY sure you're in no danger of being ganked.
Once you have 1-2 points in your Q and W, you can start taking down the enemy's health more potently. Harass with Q whenever you can (don't sacrifice farm to do so, though). If your lane is adequately warded and / or you're not scared of the enemy jungler attacking, you may use the following combo:
Shadow Dash to your opponent. Once they're hit, quickly Q them and start autoattacking. You should be able to sneak in a Ki Strike and a regular autoattack or two.
If the enemy is a mage, wait for them to recover from the taunt, use Feint to block any counter harass, and throw in another Vorpal Blade if possible.
If the enemy is an AD assassin, you may want to activate Feint as soon as you taunt them. Then Q and autoattack.
At level 6, you need to be doubly careful. Your enemy has access to their ultimate ability, which is most likely much stronger than yours in a 1v1 fight, and your allies may need your help with your own ultimate. Watch the map and don't get into fights you can't win.

MID GAME:
Your gameplay won't have changed much. Teleport or Stand United where you're needed, but be careful not to leave your own turret vulnerable. In a fight, a taunt backed by Feint and a Vorpal Blade along with a few autoattacks on a squishy target will take a lot of heat off your team while most likely killing or gravely wounding the enemy squishy as well.
If you get a gank while your enemy is quite pushed and lacking an escape ability, you could Stand United to your jungler (or whomever else is ganking) while they're still hidden. Your opponent will think you've left the lane to help others and start pushing, only to be attacked by the both of you. A well-placed Shadow Dash (which will now be going towards your turret as well) will most likely secure the kill.

LATE GAME:
The big teamfights start to break out, and assassin Shen's job is much more complex than a regular tank's.
You generally want to be around your team, so you don't have to use your ultimate to get to them. That's because, factoring in the channel time, you'll either waste the shield before it is needed, just to get there, or you'll get there too late to help.
In a teamfight, you will generally be targeting the most dangerous enemy squishies. Once you have a target, Shadow Dash to them (catching as many enemies as you can in the taunt as well). From then on, activate Feint and use Vorpal Blade on the target. Keep autoattacking and using Vorpal Blades as your team engages. This will make quick work of most squishies, essentially starting the fight with a 4v5 in your favor.
When your health gets too low, the fight is far from over. Fall back, let your team do the heavy work, and wait until one of them is in danger. Use your Stand United on them, saving them from death, and emerge with renewed energy, taunting and attacking priority targets with whatever health you have left. Two AoE taunts and two squishies taken down quickly, as well as an enormous shield (giving a teammate a second life, essentially) will pretty much guarantee your victory.
Also, it is notable that a champion as mobile as Shen with Teleport and quite noteworthy attack speed can help with splitpushing ( Ki Strike also works on turrets!), while still being able to help his team if necessary. Note, however, that you will be left without Stand United if you use it to reunite with your team, and it might be better to reserve it for the best moment possible.
Of course, if a teamfight does break out and you're not around, it is unquestionable that you should use your ulti to join your team, barring extreme cases (such as when an enemy is missing from the fight as well, and you can take that time to take down a turret or two while still leaving your team relatively safe).

That's pretty much it. Win teamfights and capture objectives (dragons, barons and turrets) and you've won already.

Here's the fun part.
Let's see, where do I begin?

We want to start off with either Doran's Ring or Amplifying Tome. You'll be needing the tome soon enough, and it is my usual start, but the ring can help if you feel you'd be in trouble without it.
While I don't recommend it, as Shen is durable enough on his own, one could use Cloth Armor against AD assassins or Boots against champions dependent on skillshots.

Your first goal should be Nashor's Tooth. Attack speed helps tremendously with your Ki Strikes and lets you do something while your Vorpal Blade and Shadow Dash are on cooldown. The ability power gives you stronger harass and shields.

Next up is probably Rabadon's Deathcap, for pretty obvious reasons.

Rylai's Crystal Scepter provides health for tankiness as well as empowering your Ki Strike and the regen from Vorpal Blade.

This pretty much sums up what you want. Attack speed, ability power and health.

On to situational items.
Wit's End can help tremendously, but is not to be purchased every time. I'd generally buy it if my lane opponent is a mage and is somewhat dangerous at least, or if the enemy team has more than one AP caster. Basically, look for excuses to use this item. If you can find none, don't use it. If you can justify buying it though, do so, because the extra attack speed and damage on hit are beautiful.
If you need more tankiness, I'd go for Liandry's Torment, since it not only synergises greatly with Rylai's Crystal Scepter, it also gives you AP and tankiness in the form of health, which helps you more than armor or magic resist, since it empowers your abilities too.
However, if you need specialised tankiness, Spirit Visage offers magic resist and makes your regen better, Randuin's Omen gives you armor and health, which we love, and Thornmail is a last resort against heavy autoattackers. I'd generally only buy Thornmail if it is absolutely necessary and only after I already have a Randuin's Omen.
Also, armor and a great way to defend against champions such as Zed are provided by Zhonya's Hourglass.
Void Staff gives you more damage and better shields. Buy it if you're ahead already and the enemies are building magic resist.

Will be added to the top as I face champions and form opinions about them. Might be a bit slow since you don't always get to play AP Shen, sadly.

While Shen can be a great tank, he has hidden potential as an assassin focus on AP and attack speed. He retains a form of silence ( Shadow Dash), which has been taken away from the rest of assassins, can burst a target with relative ease, is tanky, helps his team as well as takes down the enemies with taunts and enormous shields, and is generally a gem if played well. Give it a try.
I appreciate any feedback, feel free to applaud me or rage on me :3

Special thanks to jhoijhoi for the Template that insired me (and parts of which i just copied and pasted). It can be found here.