Your adversaries could have the mightiest defenders, but it's of little use when attackers are unblockable. For those looking to win via combat damage, few options are as appealing as evasion, allowing your assault to waltz right by blockers. And maybe swindle some loot while you're at it.

Looter il-Kor by Mike Dringenberg
In this episode of The Mechanics of Magic, we'll part the veil of reality to explore the incorporeal. Well, at least incorporeal to other creatures, as attackers with Shadow have zero issue punching opponents. Those interested in winning via combat and/or saboteur abilities will find considerable interest in this bizarre band of reality-warped troops, as 99% of the time, they'll be completely unblockable to all opponents at the Commander table.
Background
Debuting in 1997-1998's Tempest Block (Tempest, Stronghold, and Exodus), Shadow not only sped up gameplay, but provided a flavorful theme for the storyline's events. Within the lore, Yawgmoth established the artificial plane of Rath as the basis for his invasion of Dominaria. We're decades before the concept of Realmbreaker, so in order to get his troops from plane to plane, Yawg's plan was to superimpose Rath directly onto Dominaria, merging realities between worlds. As his control over Rath grew, so too did the plane itself, occasionally making 'blips' into Dominarian reality. Those poor Dominarian souls caught in this events would suddenly find themselves transported to Rath, unable to return home. For some, this new existence proved harsh (Nomads en-Kor, Paladin en-Vec, Chieftain en-Dal), as they resisted the Envicars, the Phyrexian-controlled ruling force that could shape the plane's composite matter: Flowstone . Others allied with the Phyrexian agenda (Looter il-Kor, Mage il-Vec, Vhati il-Dal), perhaps unaware of how insidious their ultimate plans were.
A rough hand to be dealt either way, but when it comes to those who received Shadowabilities, things were even worse.
Three mysterious groups - the Soltari, Dauthi, and Thalakos - originally hailed from Dominaria, but during a planeshift with Rath, were caught between worlds rather than teleported. This existence in the void allowed them to see both realities, but rendered them largely unable to communicate with either. The isolation drove all but the Soltari went insane, due to their strong religious beliefs and a hope that they'd one day become corporeal again. Lyna Soltari, depicted in non-Legendary form as Soltari Emissary allied with the story's Weatherlight heroes to help prevent the invasion and restore her people to reality. Though they were ultimately unsuccessful at stopping the invasion (Invasion, Planeshift, Apocalypse), Dominaria was able to defeat Yawgmoth.
Jump ahead to Time Spiral Block (Time Spiral, Planar Chaos, and Future Sight), and Shadow made a return (Augur il-Vec, Shadow Sliver, Faceless Devourer), much like how Flanking was also brought back in this era. This time, various time rifts have pulled centuries-old inhabitants of the old-Rath of the past to Dominaria's post-apocalyptic present, along with peoples and creatures from all over the plane's history. Lost and bewildered, these being seek to survive in an unstable world of historical misfits. Tivadar of Thorn must look awfully confused underneath that mask.
Rules
Creatures with Shadow are essentially trapped within their own combat zone, only able to block and be blocked by creatures that also have Shadow. If your Soltari Trooper is facing an opponent with no Shadow creatures of their own, he's unblockable. However,Soltari Trooper can only block other creatures with Shadow, so it's of little defensive use if an opponent sends something scary like Krosan Cloudscraper your way. You'd have to hit the incoming monster with a Shadow Rift before your trooper could chump block. These scenarios illustrate that Shadow is both a blessing and a curse. On one hand, creatures with the ability are highly evasive, as few other creatures possess Shadow. Conversely, this also means creatures with Shadow are unable to block 99% of the time. A Soltari Priest would laugh in the face of an attacking Zurgo Helmsmasher...if only he were able to block the Orc, that is. Protection from Red may stop a Lightning Bolt from axing the priest, but it's not going to help with the 7/2 Commander charging at your throat.
To get around this, Wizards printed a few tools to help turn Shadow on and off. Soltari Emissary, Trespasser il-Vec, and Thalakos Drifters can gain Shadow when attacking, but you can opt not to activate their ability when in need of a blocker. Dauthi Embrace, Traitor's Clutch, and Dauthi Trapper take the opposite approach. They can grant your own creatures Shadow for added evasion, but if a blocker is needed, can also target opponent's creatures, giving them Shadow so you team of Dauthi can intercept. Phyrexian Splicer is another fun tool, able to swap around abilities, Shadow included, from one creature to the next. The Splicer can target enemy creatures, making for some interesting political applications in Commander.
Unblockable much of the time, Shadow creatures are excellent vehicles for saboteur abilities, which trigger once a creature deals combat damage to an opponent. Soltari Guerillas, Thalakos Dreamsower, and Soltari Visionary all act as removal engines. Thalakos Betrayer takes things a step further with his impression of Control Magic. Dauthi Mindripper will tear a sizable chunk of your opponent's hand size after a toothy bite. Those fangs look nasty.
Commander Options
Despite Lyna Soltari being a key side character in the story, there currently exists no dedicated Shadow-general among the Soltari, Thalakos, or Dauthi. As such, a Shadow-themed deck has to consider other legendary creatures that benefit from routinely sneaking through for damage. One option is Ninjitsu, seen on Commanders like Yuriko, the Tiger's Shadow and Satoru Umezawa. With these at the helm, you can bet your army of Dauthi and Thalakos will easily be able to connect, sowing Ninja-shenanigans along the way. Expand your horizons into Green and you gain access to Cazur, Ruthless Stalker and Ukkima, Stalking Shadow, which will bulk up your army of attackers for greater damage as the game progresses.
For the Soltari, we need to branch into Naya. Marisi, Breaker of the Coil's Goad ability triggers for each player dealt damage by one of your creatures, allowing a trio of Soltari troops to replicate Disrupt Decorum each of your turns. Why bother smiting opponents when it's so much easier to let them run over each other? If you're feeling Tolkien, Aragorn, Hornburg Hero plays a similar role to Cazur, bulking up your creatures once they deal damage, but rather than growing one counter at a time, Aragorn's buff effect scales exponentially each combat. The Soltari might start out small, but they won't stay that way for long.

Soltari Visionary by Adam Rex
Melancholic as he may be, that's an excellent ability to pair with Shadow!
Here's hoping today's shadowy showcase gives you ideas for future Commander decks. Though only about 43 creatures currently have the ability, recent cards like Minas Morgul, Dark Fortress illustrate that we haven't seen the last of it. Now that Omenpaths stretch across multiple planes, allowing anyone to travel from world to world, I wouldn't be surprised if the occasional traveler got stuck in-between.
Thanks for reading, and may you never get stuck between worlds,
-Matt-