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