I think that the mythoclast was created by the vex and given to the guardians to eventually destroy them. They knew that the weapon would be used in the crucible and that it would cause infighting and dissent amongst the guardians. The intent was to cause guardians to spend their energy squabbling in the crucible, which would distract them from fighting the darkness. Additionally, they foresaw that the infighting would result in the vanguard deliberately weakening guardians' weapons and powers. The next step in their plan to destroy the guardians was to tempt Osiris with hidden knowledge. They deceived him into starting a separate tournament, which served to further deepen the vanguard's resentment of one another.