If you go to player-title for EoF and look at the triumphs it should show if you missed a mission on Legendary.

Play the [b]Sieve[/b] Activity on Kepler. After the boss is defeated at the end of a given round you will get a period of “bonus time” in which you must defeat combatants and bank the motes they drop at the central platform. If the group fills the meter you will be rewarded with three chests; the biggest of these chests appears to be a [b]guaranteed[/b] drop for the new Exotic for your Class, provided you have completed the Legend Forged Triumph. (΄◉◞౪◟◉｀)