Hey, I'm a Talon and Shen main on the NA servers. I started playing in Season 5 when Ekko was released and haven't stopped since.

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Pros
-Energy Based, doesn't need to recall due to mana ussues
-Tanky sacling with his passive
-Has a gap-close and taunt
-Has very high damage output and all in potential
-AOE AA blocking shield
-Grants a Massive shield from ult
-Global pressure from ult

Cons
-Based on landing Taint
-Dependent on items
-has long cool down timers
-no heals
-require you to watch map and protect adc at the same time

Ki Barrier

Shen's Passive, Ki Barrier, is pretty straight forward, you use an ability, you get a shield that scales 47 + (3 x level) (+14% of bonus Health) for 2.5 seconds

Twilight Assault

Shen's Q, Twilight Assault, is his main damage output. Activating it calls his spirit blade towards him and does 2 / 2.5 / 3 / 3.5 / 4% max health damage for the next three auto attacks. Calling the blade through an enemy increases the 4 / 4.5 / 5 / 5.5 / 6% max health damage and gives him 50% bonus attack speed. Calling a blade through an enemy also slows them by 35%

Spirit Refuge

Shen's W, Spirit Refuge, is his main defensive spell. It creates a defensive circle where his spirit blade it, blocking auto attacks for 1.75 seconds while in it.

Shadow Dash

Shen's E, Shadow Dash, is his main form of CC and engage. He dashes to a target location, dealing 50 / 85 / 120 / 155 / 190 (+12% of bonus Health) and taunting them for 1.5 seconds.

Stand United

Shen's Ult, Stand United, is what makes Shen who he is. After channeling for 3 seconds, he teleports to an ally shielding them for 250 / 550 / 850 (+130% of ability power) for 5 seconds.

The runes themselves aren't anything special. Run aftershock, as it will give bonus resistances when you engage, and based on the all-in play style, it synergizes pretty well.
Guardian is okay too, but I don't prefer it.

Other than that, it's really up to you, most of the runes in that tree are viable. Personally, I run demolish for the extra tower pressure, chrysalis because it makes you tankier early, and overgrowth because it'll scale well later into the game. Bring precision as a secondary, as Alacrity scales well with Shen.

work in progress

work in progress