Playing Shirei has been a fun take on mono black, and imposes some interesting deck building constraints that keeps gameplay fresh. I've found the most powerful strategy focuses around recurring ETB and death triggers from small dorky creatures. The other option is a Shadowborn Apostle route, but this tends to be more of a one-trick pony and opponents will see what you're up to right away.
-Playstyle and Caveats-
This deck can be absolutely oppressive if you play in a creature heavy meta and has no problem keeping your opponent's boards clear of creatures, even against some of the more powerful token decks. Since you will be saccing creatures for value on each of your opponent's turns, Shirei's power scales with how many players are in the game. 4+ players is the ideal environment for this deck to shine.
The first and most important goal in every game you play with this deck is to cast and protect Shirei. At first most people don't threat assess this deck properly and will leave you alone. However, as soon as you start tearing their hand apart, making them sac all their creatures, or drain them out with Blood Artist, you need to be much more conservative about when you choose to play Shirei. This deck will struggle in the late game without the constant recursion Shirei provides, so finding a safe window to play your commander, with protection back up in the same turn, is the biggest hurdle to overcome. If Shirei is removed, often it is best to let them go to the graveyard so that you can recur them back to your hand or even straight to the battlefield rather than paying the extra commander tax.
The second goal is to get a cheap and repeatable sacrifice outlet onto the board so that you can sacrifice your own creatures on both your turn and your opponent's turn. The best sac outlets are free, such as Bloodthrone Vampire or Spawning Pit; we'd play a free sac outlet with no effect in this deck if we could. The worse sac outlets I play in this deck force you to tap or cost 1 or 2 mana to activate such as Priest of Forgotten Gods or Vampiric Rites. In general these are only played if they can help you find better sac outlets.
Once we have Shirei and a cheap sac outlet, our third goal is to deploy a small army of creatures with death or etb triggers. Be sure to sac everything you can at the end of the second main phase on every turn, or as often as you can. Sacrificing on the end step is what we're trained to do as Magic players, but given Shirei's wording, it won't work with this deck. Also be wary of effects that pump your creature's power. If they have more than 1 power when they die (like with Skullclamp on a 1/1), Shirei won't bring them back. That's why most "Caged Sun" effects aren't included in this deck.
If you can get Shirei out and protected, there are a ton of value plays you can make to refill your hand and tax your opponent's resources. The subtlety lies in deciding how much to commit to the board, and how many cards to keep back in your hand to redeploy if your opponents ever find a way to bypass Shirei's protection. You don't need a ton of cards on the battlefield to put yourself in a winning position, so often it's correct to hold off on playing more stuff so you don't get blown out by a board wipe or Shirei spot removal.
This version of the deck is a little greedy because it only runs Scour from Existence as a way to deal with problematic non-creature permanents. One Rest in Peace or similar effect from your opponent can completely shut you out of the game so I usually save general permanent removal specifically for those silver bullet cards. I think its completely justified to run Meteor Golem and Spine of Ish Sah, but my playgroup happens not to run many non-creatures that completely shut me out, so I chose to lower my curve instead.
Overall, Shirei has been super fun so far, and almost every set has some cool new black common that might make a spot in the deck, so there are tons of cards to try!
