A funnel is an introductory adventure for a large number of characters, designed to kill many of them. The metaphor of the funnel, wide at the head and narrow at the tail, refers to the adventure’s lethality. Typically, characters are common people thrust in highly uncommon (and dangerous) situations. Mechanically, this is simulated by dice-based randomization of attributes, few or no useful pieces of equipment, and very low HP. Simultaneously, the funnel GM is often encouraged to not hold punchs and have damage and death in store the PCs at every turn. Whereas a traditional starter adventure might be a gradual ramping up on challenge, the funnel is intended to be unforgiving throughout. This affords the GM the opportunity to show off gameplay systems without risking a player being removed from play. The intended effect is to give a sense of danger in the world, demonstrate from the outset of play the worth of the PCs who survive the funnel, as only the survivors rise to next level.

Dungeon Crawl Classics

The 0-level character funnel is a built-in character creation mechanic of DCC, and widely associated with its gonzo house style. Here, players create 4 characters each. After running them through the funnel, player choose one character amongst their survivors (if any) to advance to the 1st level and start their life as an adventure. The mechanic gives each new character some backstory (often emotionally charged), connections to the other PCs, and an event that distinguish them from the common people.

Further Reading

For widely-lauded examples of funnels in print, see the DCC modules Nebin Pendlebrook's Perilous Pantry and The Sailors on the Starless Sea.