Here is the full list of all 15 The Grave Digger achievements.
Complete the tutorial graveyard
Collect all treasure in all graveyards
Find all the Lost Keys
Be chased by 5 or more ghosts at the same time
Complete a level undetected
Complete a level without lighting the beacon
Spend over 5 minutes looking at graves
Complete a graveyard between 12 AM and 4 AM
Collect the golden shovel
Avoid a chasing Bride using a beacon
Collect all items of clothing in the game
View the developers headstone
Complete all graveyards on Easy
Complete all graveyards on Normal
Complete all graveyards on Hard