Ever since the Edge of Fate update there is no clear way to obtain the achievement/trophy Challenge Accepted (complete 30 challenges), The Prestige (complete a Nightfall strike on Grandmaster difficulty), or Darkness Falls (Defeat a Forsaken Nightfall Boss). Please either update content in the game or the achievements themselves to make them obtainable. Right now the only players unlocking these are ones who met the criteria prior to the update and cross-loaded their save on another platform.