After you are done loading the brand new Destiny Dark Below DLC you will see that the weekly Heroic and Nightfall missions will not be available until you “complete the new quests.” Go through the guide given below to do exactly the same: Unlock Destiny Dark Below DLC Weekly Heroic and Nightfall missions by beating the three story missions, the “kill 25” quest and all three purple Earth Patrol missions.
The first phase of unlocking the Nightfall is completing the three story missions by the new NPC Eris Morn, and get the “kill 25 Knights or Wizards” bounty. You need to complete the same quest on Earth two to three times to finish them. If you are interested in getting more Knights and Wizards then just delay the delay the death of the boss.
Now complete the new set of quests that Eris will produce. On the planet Earth chose Patrol and navigate your way through The Divide and past the Rocket Yard. You will feel like you are playing the Sepiks Prime strike. Find the Hive enemy in the yard after the tunnel, destroying the Hive enemy will give you your first bounty.
To get your second Bounty, start the Earth Patrol once again and keep going north and up the hill into the complex to find the Wizard room. Look for Thrall in the corner and finish it. Now go past the Wizard room to find a loot cave and locate the big open area of the Terrestrial Complex beside it. Get the urn from the small landing while destroying the enemies that come in your way.
You should now be able to unlock the new Strike and the Weekly Heroic and Nightfall missions. Watch the video given below for more information: