rahool is the cryptarch. There was once a time when many guardians would fire countless rounds of ammunition into that cave, slaying hundreds upon thousands of unfortunate hive. The guardians would then collect the engrams that the hive would drop, and return them to master rahool in hopes of some epic loot. Well master rahool almost always disappointed at that time. Hence even a million hive corpses would not satisfy him. They put that little easter egg in that cave after they patched the cave from being the "loot cave".