The Delving of Fror is a zone that can be entered by a side when they control at least two of the five outposts in the Ettenmoors.  Once you have entered the zone, you are not removed from the Delving if your side loses control of at least two outposts, but cannot re-enter until the required number of outposts are retaken.  Because there are five outposts on the map, both sides can be in the Delving at the same time.  Unfortunately, PvMP combat in the Delving happens very infrequently, mainly because the zone does not contain sufficient rewards for freeps to be there on a regular basis.  Furthermore, while all mobs in the Delving are hostile to both sides, they proc various debuffs which only affect freeps.

The overall map of the Delving is below, with the creep entrance on the left and the freep entrance on the right.

The Delving's primary trophies for the creeps are the Dull, Glimmering, and Luminous Spirit Stones.  Creeps can use them to purchase enhanced corruptions, skills, and pots, while the rewards for freeps are less significant.  The Delving is not a very significant source of DP.

The Dull Spirit Stones drop off of any normal level mob, which are found throughout the Delving.

The Glimmering Spirit Stones drop off of any signature level mob.  The two major farming zones are the blue Worms in the very top central part of the map on both levels and the red and black Salamanders in the upper half of the central area.  The Salamanders are best farmed by a fellowship or small fellowship, as they typically appear in groups of three and hit pretty hard.  The other signature level mobs in the Delving are generally too annoying with stuns and power drains to farm.

The Luminous Spirit Stones drop off of any Elite or greater level mob.  Because the various Elite Masters and the single Nemesis-class monster in the center of the zone drop Luminous Spirit Stones at the same rate, there is really no reason to defeat the latter except out of curiosity.

There are also Brilliant Spirit Stones, which drop from a couple of the higher-level mobs in the Delving.  Freeps can barter them in combination with other stones for various (rather terrible) rewards, but at present they are entirely useless to creeps, as they cannot be bartered for anything.

The map below shows the names and locations of the various higher-level mobs in the Delving.