The Delving of Fror is a zone that is open to a side when three keeps are held by them.  When the controlling side loses a keep, they are not removed from the Delving so the new controlling side may enter and engage in PvP action.

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.  These are used to purchase enhanced corruptions, skills, and pots for the creeps. 

The Dull Spirit Stones drop off of any normal level mob which are found all over 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 Salamanders on the upper half of the central area.  The other signature level mobs are generally too annoying with stuns to farm.

The Luminous Spirit Stones drop off of any Elite or greater level mob.  There is no differentiation in drop rate from the Nemesis in the middle to the 44k Elite Masters, therefore there is little/no reason to defeat the more difficult bosses except out of interest.