From the Introduction:
When your PCs gain access to the teleport spell, their whole world changes. That simple spell opens up instantaneous, long-distance travel. No more long overland journeys or dangerous retreats through hostile territory. All it takes is clasped hands and a word from the wizard, and poof! The PCs are where they want to go.
Except that it isn’t that simple, because teleport isn’t foolproof. The off-target teleports are a matter of scatter- ing your PCs someplace else on the map and forcing them to get their bearings and make the long overland journey anyway. But this Side Trek focuses on the really intriguing column on the teleport chart: “similar area.”
Save this Side Trek for the next time the PCs try to teleport and wind up in a “similar area” rather than the place they were trying to go. In this case, the similar area is an outpost of the mind flayers of Thoon (found in Monster Manual V). If the PCs were trying to teleport back to their home city or to a temple where they could acquire healing—two of the most common teleport destinations—they can wind up in the clutches of the mind flayers instead.
This Side Trek is designed for 10th-level PCs, but it works well for any group that has access to teleport but isn’t using greater teleport (which doesn’t have the possibility of a “similar area” result).