To unlock the Grave Robber trophy in Mortal Kombat 11, you will need to open 200 Krypt chests.
Krypt chests require Koins to open them, with the amount required ranging from 2,000 to 10,000+. Sticking to the lower priced chests is the cheapest option, and will require approx. 1,000,000 in total.
You earn Koins after every match in Mortal Kombat 11, with bonus Koins added if you finish matches with a Fatality or Brutality. The quickest (also the hardest) method to earn the required Koins will be to play the Towers of Time for the large Koin rewards. Alternatively, play the Klassic Towers if your find the Towers of Time to be too difficult.