Create an entry in
Loot/Sets.txt. This determines which item will be dropped, how many etc. The fourth column is the item ID of the essence, the third column is chance that the item will drop. 300 can be thought of as a 300% chance chance, i.e. 3 essences will always drop (as a rule of thumb, one essence per player in recommended party size for the associated quest)
..
essence_darkdepths_vitiator 0 100 20532 Vitiator's Essence
essence_direwolfden_foulfur 0 100 20778 Foul Fur's Essence
essence_folkirkmanor_fathergrundy 0 300 10015 Father Grundys Essence
essence_djinntemple_lordafrott 0 100 20769 Lord A'Frott's Essence
essence_fangarianslair_grimfrost 0 110 21299 Grimfrost's Essence
..