Macbeth, also called The Tragedy of Macbeth, is one of William Shakespeare's most famous plays, written around 1606. It depicts a distorted account of Macbeth of Scotland and his rise and fall. In its cast of characters, it includes Macbeth himself, as well as King Duncan and the Weird Sisters. The character of Lady Macbeth, while ostensibly based on Gruoch, shares more traits with Demona, the "other woman" in Macbeth's life.
The play portrays Macbeth as something of a villain and Duncan the noble king (a reversal of their roles in the Gargoyles Universe), although this varies according to the specific production. The play is not historically accurate; Shakespeare probably changed the events to avoid offending King James I of England, a descendant of Duncan.
Macbeth is familiar with the play, but amused rather than offended by it - partly because he and Shakespeare were close friends.  His amusement of the play's bad history also allowed him to appreciate the truths revealed in the play, even if they weren't his truths. Macbeth also wasn't too upset at the portrayal of Lady Macbeth, seeing no resemblance between Gruoch and the boy playing his wife (it also helped that Shakespeare never utilized her true name. 
Before the termination of the Gargoyles series, a two-part episode was planned called "The Weird Macbeth", in which several Gargoyles characters were trapped in a production of this play. It would have had Macbeth playing himself, Demona as Lady Macbeth, Goliath as Macduff, Elisa as Lady Macduff, and Hudson as King Duncan. The higher-ups at Disney would not allow the episode to be made as a two-parter, and feeling that it was too complicated to fit into a single episode, Greg Weisman dropped the notion of including it in the second season of the television series. 
According to a common thespian superstition, it is bad luck to utter Macbeth's name in a theatre - except as appropriate while on stage and during a production of the play itself. Thus, it is usual for theatre workers to euphemistically refer to the tragedy as The Scottish Play. The general belief is that if any person utters the name "Macbeth" inside a theatre, the play currently in production will suffer some calamity or even fail entirely. Stories of bad luck, botched performances, property damage, injury, and even death resulting from speaking the play's name circulate among thespians. Fortunately, there is a short ritual, which involves spinning in place and praying, which can avert the danger if used immediately after naming the play.
Supposedly, the play itself was cursed by witches, who became angry because Shakespeare used real witch spells in his text. Because of this curse, not only is the name dangerous to utter, but some consider it unlucky to produce the Tragedy of Macbeth, and believe that it will bring misfortune upon a theatre house. It is worth noting, however, that the actual religion of witchcraft opposes any use of magic curses.
- Macbeth (play) at Wikipedia, the Free Encyclopedia