From Introduction:
In From Beneath the characters pass over ground occupied by the undead. As wanderers and adventurers, the PCs will frequently be traveling overland, so this encounter can be fitted easily into most campaigns. Often, the pressure to complete or continue the main thrust of the campaign means that, when the players set out on these journeys, time passes and they "just arrive" after some bookkeeping by the DM. Critical Hits are designed to fill these moments with opportunities for exciting roleplaying and to force the players to take nothing in the campaign world for granted. We have kept the descriptions of location and place names to a minimum so that the encounter will fit easily into almost any campaign. Feel free to change any details that don't fit your world. The only really important aspect of the encounter is that the PCs encounter the undead on barren land, so all your game world needs is a patch of ground!
This entire encounter is for those PCs who just want to wade through piles and piles of undead until they are totally satisfied!
Having dealt with each one of these undead encounters, the PCs experience the anger of the evil land. In From Beneath the evil manifests itself as a series of dire obstacles; each one as deadly as the last; each leading to the next combat encounter. So ready those holy symbols and don't let them draw breath until this slugfest is over!