Sidisi, Undead Vizier (EDH / Commander)

It's a pretty straightforward deck. Punish people who attack you, and control the game until you're ready to kill with:

Basalt Monolith + Rings of Brighthearth + Sensei's Divining Top/Staff of Domination/Praetor's Grasp or Helm of Obedience + Leyline of the Void + Sculpting Steel
Helm of Obedience + Leyline of the Void
Cabal Coffers + Deserted Temple + Rings of Brighthearth + Sensei's Divining Top/Staff of Domination/Praetor's Grasp or Helm of Obedience + Leyline of the Void + Sculpting Steel

Your best tutor target is usually Cabal Coffers, Urborg, Tomb of Yawgmoth, Deserted Temple, Vesuva, or Candelabra of Tawnos, as these cards combine to generate an insane amount of mana in this deck. Later in the game, Yawgmoth's Will or Diabolic Revelation is your best bet.

Sidisi, Undead Vizier is a deck that relies on subtlety and your opponents being more afraid of each other than of you. There's a saying in my EDH group: "The third player to be scary wins the game." Basically, the first person who does something insanely powerful gets hated out by the other players, the second player to do so gets whatever is left of the counters/removal, but by the third time someone tries to win the game no one has enough cards left to stop them. Your goal with Sidisi, Undead Vizier is to be the third player who tries to win the game.

The best way to play is to sit back while building up cards and mana for the first six to eight turns of the game. Try not to do anything overtly threatening until your opponents have blown their loads. Tutor for cards like Cabal Coffers that set you up for the long game without drawing attention to yourself. Let your opponents over-commit to building up their board state then blow an Oblivion Stone or cast All is Dust or my personal favorite: Decree of Pain. All those extra cards you drew and all that extra mana from Cabal Coffers should leave you better off than everyone else. If you're going for an instant kill like Helm of Obedience + Leyline of the Void, play and use it in a single turn whenever possible. Leaving half of a kill out there will draw attention to yourself and make your opponents target you every time you play a tutor.

As the speed in which your playgroup kills varies, you're going to have to adjust the deck to continue being the third scary person.