Bottomless Pit (CR 9)

A bottomless pit is a yawning chasm that appears to be a perfectly natural fissure in the earth, other than the fact that it appears too deep to see the bottom. In actuality, bottomless pits are entrances to extraplanar spaces filled with nothing but endless empty void. A creature that falls into such a pit (whether it is pushed, runs afoul of a trap that conceals the bottomless pit, willingly dives into it, or enters the pit in some other fashion) falls endlessly in inky darkness at a rate of 500 feet per round. Other than its depth, the extraplanar space’s dimensions match those of the pit’s entrance, and the falling creature can attempt to catch itself on the wall, using the Climb skill (DC = 20 + the wall’s Climb DC, as normal for catching yourself when falling), and can attempt to climb out of the pit from there. The wall’s climb DC typically matches the type of terrain the opening was in, so a rocky chasm has a Climb DC of 15, for example. The creature can attempt to catch itself once per round. Because the creature falls endlessly, it can rest and even prepare spells while falling (although it must be careful not to drop any possessions, since it will likely fall at a different speed than the possessions do, causing it to lose them forever).

If your campaign uses the sanity rules, for each hour a creature falls in the bottomless pit, it must succeed at a DC 20 Will save or take 1d4 points of sanity damage.