The Order of Underworld Travelers (2007)

A One-Round Living Greyhawk Gran March Regional Adventure

From the introduction:

More than a year ago, hundreds of tons of rock collapsed into a cavern below Shiboleth filled with daemons, killing them and destroying their gateway from Gehenna. Ancient vampires who were caught between the Gran March army and the daemons were also crushed by the collapse. Some folks are concerned that the threat from one or both of those sources is not truly crushed. No one really wants to go into the muddy, dangerous, shifting ground beneath the flooded sinkhole to investigate. Volunteers are being accepted.

A Gran March Regional adventure for [character levels] 2 - 12.

Part Four of the Shattered Reflections series.

Note: This adventure will be of particular interest to members of The Order of World Travelers, Syrloch, The Gran March Army and Knights of the Watch. [Characters] who played GRM6-01: All That is Secret and Hidden or GRM6-07: Athenaeum who are members of a recognized Gran March meta-organization or Sheldomar Valley meta-regional meta-organization would also be requested by that organization. Large size [characters] and companions might experience additional difficulties with this scenario.