The Tomb of Horrors is a D&D module that one can argue lays claim to the title of "first" D&D module (see my node on Palace of the Vampire Queen for a discussion on "first"). It was officially released in 1978 and authored by Gary Gygax. This was a full two years after TSR's first official module release: 1976's Steading of the Hill Giant Chief. However, Gary Gygax himself makes reference to it as the first. Gygax's claim lies in the fact it was the first TSR module to make a public d├ębut. It was a tournament module used at Origins I (1975).

Tomb of Horrors was a decidedly high-level adventure. It was intended as a bit of the ol' chin music, an attempt to cut players within Gygax's campaign down to size. Hence the lich encounter at the end. However, unlike dungeons of the time, Tomb of Horrors featured very few actual monster encounters (save for that rather nasty lich encounter at the end). In the early days, there was a rather nonsensical approach to dungeon design. It was room after room of monster encounters. Frequently referred to as a either a dungeon crawl or a "monster hotel". One room had orcs. The next room had a wyvern. The room after that had trolls. None of the monsters seemed to take notice of each other or ever leave their rooms to forage for food. And some of the monsters were of such large size, one wondered how, say, a red dragon squeezed his way down a 10'x10' hall. Their existence seemed to be predicated solely on the idea that if they waited in their dungeon hotel room long enough, a tasty band of adventurers would present themselves.

The main challenge in Tomb of Horrors was its various traps. Any player can reach level 12 slaughtering orc after orc. However, any player worth his double digit character level should have a level of intelligence and guile to match. Tomb of Horrors tested that assumption. It required players to figure out a series of traps. Failure meant instant death or some cruel, cruel twist, like changing the character from a man to a woman.

Tomb of Horrors also had this odd "innovation". It had a 12-page picture booklet. Certain rooms were keyed to certain pictures in the book. When the players entered a room, the DM was queued to show the characters the appropriate picture. The art work ranged from rather terrible to classic. The better illustrations being produced by Wormy creator David Trampier. It had a certain "smell-o-vision" quality about it. Like smell-o-vision, the keyed illustration gimmick never really caught on.

Tomb of Horrors was given the module code S1, the first in the TSR's S-series. S stood for "special". Each were stand alone modules, tending toward high level, and usually found their start as tournament modules at Origins or GenCon. The other three in the series were:

S2 White Plume Mountain by Lawrence Schick
S3 Expedition to the Barrier Peaks by Gary Gygax
S4 Lost Caverns of Tsojcanth by Gary Gygax

Two more S modules were planned, S5 Dancing Hut of Baba Yaga and S6 Labyrinth of Madness, but did not actually come out until 1995, long after TSR changed their module numbering system.

S1-S4 were packaged into a "super module" series called Realms of Horror.