Nightfall missions will unlock at around level 24-25 but require a player to be at least level 28 to complete. They will appear in the orbits menu on the selecting planet screen, they appear at the left hand side of the screen. These nightfall missions are currently the hardest content in the game at this time. :)

Nightfall missions are currently the hardest bit of content in the game until the raid is released. The loot rewarded is more legendary than you are.