Searching your library is the greatest thing you can do in Magic. I challenge you to find a player fanning through his or her deck in the middle of game without a great big smile on their face. It’s because they’re all thinking the same thing! “Look at all these great cards, I’m gonna find the perfect one and I’m gonna win with it!”
Whether it’s Stoneforge Mystic, Birthing Pod, Chord of Calling, or just a Windswept Heath, you know you’re usually going to get exactly what you want. With Sidisi, Undead Vizier, you’re always going to get exactly what you want. Search your library for any card and put it in your hand. Any card!
I remember playing with Diabolic Tutor in Constructed. The absolute nightmare scenario floor for how bad Sidisi can be—when you have no other creatures and would have to exploit herself—is likely still better than Diabolic Tutor because you can choose to simply have a 4/6 deathtouch.
On that topic, Sidisi’s combat stats are outrageous! For an effect like this, we’re lucky to get a vanilla 2/2, but a 4/6 deathtouch beats (or at least trades with) almost anything in combat, and does a great job stabilizing the board to ensure you survive to cast whatever card you tutored for.
I expect Sidisi, Undead Vizier to be a major player in Standard and to likely make an appearance in Modern as well.
She’s strong enough to play in any deck, but scales in power level the more you’re willing to build around her. My first thought (my first hope!) is that she might allow a Mono-Black Control archetype to make a comeback. The best thing I can think of to tutor for on turn 5 is a card that creates a giant life total swing like a Corrupt or, in the case of Standard, a Gray Merchant of Asphodel.
Sultai Emissary and Black Cat are admittedly low power-level cards, but they’re defensive bodies that will buy you time, contribute to devotion, and go well with the sacrifice theme of this deck. Similarly, Disciple of Phenax is mostly about the devotion and the enters-the-battlefield effect, but also ensures that you’ll always have a random dude around for when you’re ready to cast Sidisi.
If you chose to, you could play eight fetchlands and some delve cards. My first choices would be Murderous Cut and Soulflayer, since it provides some nice devotion. I also like the fact that Sidisi, Undead Vizier provides a hard-to-find keyword ability in deathtouch to boost the power level of your Soulflayers.
If you’re looking for something more tried and true, this might be up your alley:
Sidisi Sidisi Sidisi Whip
Satyr Wayfinder, aside from being one of the best cards in Standard, is the king of the “random disposable body.” Sidisi, with her powerful enters-the-battlefield ability, is also right at home in a Whip of Erebos deck. Tutor for the Whip itself, tutor for an answer card, tutor for a Hornet Queen, or tutor for whatever silver bullet creature most pleases you.
In fact, tutor for any card you like…