Create a new CreatureDef for the chest itself

[ENTRY]
defHints=8
ID=7806
will_regen=1
might_regen=1
appearance=p1:{["a"]="Prop-Loot-Boss_Chest1"}
level=1
display_name=Father Grundy's Treasure
profession=1
creature_category=Inanimate
rarity=1
ai_package=nothing

Create a new ItemDef for the creatures essence

[ENTRY]
mID=10015
mType=7
mDisplayName=Father Grundy's Essence
mIcon=Icon-Icon-32-Wall_Torch1.png|Icon-32-BG-Green.png
mIvType1=1
mIvMax1=20
mIvType2=8
mIvMax2=1
mLevel=1
mBindingType=1
mValue=1
mQualityLevel=2
mMinUseLevel=1

Create the reward items you wish to be in the chest.

Create an entry instance Instance/[InstanceID]/EssenceShop.txt. Use the CreatureDefID and ItemID for the chest creature and essence item above. Also add in all the reward items you wish.

; Father Grundys Chest
[ENTRY]
CreatureDefID=7806
EssenceID=10015   ; Father Grundy's Essence
Item=10006,1  ; Grundy's Organic Hammer 
Item=10007,1  ; Grundys head band

Now create a an entry in Loot/Creatures.txt. Locate all the other loot creatures that start with essence_, and insert your new one nearby. The first number in this table is the creature Def ID of the creature that will be dropping the token. In this example, Father Grundy.

.....


975	0	essence_darkdepths_vitiator	;Aram Norr
1261	0	essence_folkirkmanor_fathergrundy	;Father Grundy      
940	0	essence_direwolfden_foulfur	;Foul Fur
711	0	essence_djinntemple_lordafrott	;Lord A'Frott
2756	0	essence_fangarianslair_grimfrost	;Grimfrost
443	0	essence_forestcavern_frietzsplithoof	;Frietz Splithoof
2340	0	essence_goliathruins_sharabat	;Sharabat
....