Heroic Mission

Heroic Mission is an event that repeats sporadically and runs for about 6 days, during the event the idea is to increase your Might, then depending on your ranking in the event you will receive a reward at the end.

But it's not just about the final reward, there are a lot of missions usually to do with increasing your Might and Normal and Advanced Map Searches which will also bring you rewards too. Check out the mission list at the start of the event.