From the introduction:
S1: Tomb of Horrors is one of the best known modules to come from Advanced Dungeons & Dragons (1st Edition), and also one of the deadliest. Written by Gary Gygax as a means of testing and challenging characters who had otherwise conquered the most difficult dungeons in a brutally unforgiving game. He delighted in carrying the module (before it was published) in his briefcase to conventions as a means of putting to the test players who bragged of “unkillable” PCs. More often than not, they left the table with torn-up character sheets, grumbling about “impossible death traps.” Every so often, however, particularly brilliant (or lucky) players would make it all the way through and have some truly epic stories to share.
This is not the Tomb of Horrors. Well, it is in a sense. Much of this game is based on the Tomb of Horrors, from its insidious theme of wit triumphing over might, to the vast majority of its devious traps. Some liberties with the source material, however, have been taken. Be warned that an unscrupulous player could find and study the Tomb of Horrors books to disarm just about every challenge inside with minimal effort. Be aware, however that this adventure is not a word-for-word reproduction from the original books.
Finally, a word on the traps themselves. This is an unforgiving setting and the devices that populate the Tomb are built by the mightiest sort of magical evil that has ever existed. The upshot of this means: There will be times when death will come without saving throws and without skill checks. While there are no “Rocks Fall; Everyone Dies” moments of DM injustice, a stray step or careless touch may well lead to unavoidable death. Not every trap is so lethal, but be aware that such dangers do lurk in the Tomb. Please remind your players that this is just a game and nobody likes a poor sport. If they fall victim to such doom: the best tonic is just to grin, shake it off, and know that you’re in good company- thousands have died to the Tomb of Horrors before and thousands are likely to die in years to come. And if this thought doesn’t comfort you, perhaps you’re playing the wrong module.