After The Wakening, Destiny: The Dark Below gets a bit freeform.
Destiny: The Dark Below guide - how to find the Hand, Heart and Eyes of Crota
When you return to the tower after The Wakening, Eris will reward you with Murmur, a Legendary fusion rifle capable of both Arc and Solar damage.
She'll also offer you two quests. One requires a purchase form Xur, so will have to wait for the weekend. The other challenges you to kill Knights and Wizards on Earth.
One way to rack up these kills is to repeat the Siege of the Warmind mission. There are also many locations on Patrol where you can fight Knights and Wizards, and if you visit all of them once you'll be well on your way to the 25 kills needed.
- From the Patrol spawn point, look straight ahead at the small shed; go inside and down the stairs. There's a Major Knight and two Major Acolytes in a dark room.
- Head to the Rocketyards. There are four Knights here, including two in a group, who respawn rapidly.
- There's a Wizard waiting for you in the Refinery, where you fight the wave battle on the Devil's Lair strike - sometimes a Knight, too.
- Just before you reach the area where you fight the wave battle in the Devil's Lair strike, there's a named Acolyte you can disturb to summon a quite tough Major Wizard. More on him later.
- In the room where you first encountered the Hive, you can fight a very low level wizard. There's a Knight on the lower level, and if you disturb the praying Thrall in the corner of the lower room, you'll summon another one, although he's quite tough. More on him later.
- Up on Skywatch there's a cave hidden on the cliff face where a Major Knight regularly spawns. There's also a Knight waiting in the tunnels on the way to the Jovian Complex, beneath the Hive Seeder.
- Watch for a message saying the minions of Crota have started a ritual. Wait a little and the Blades of Crota will arrive. They include a Major Knight with a sword you can loot once he's dead for 60 seconds of great fun.
Once you've managed this feat, return to Eris to be offered three more quests requiring you to track down specific Major Hive enemies. Note that all must be located within Patrol missions. Go back to Patrol on Earth, and track them down as follows:
Where is The Hand of Crota?
Head up towards Skywatch through the Lunar Complex. Once you pass the two Dregs and enter the room with the Knight and loads of Acolytes, look in the corners of the room; if you're going upstairs, you've missed it. There's a passive Thrall here called the Servant of the Hand.
Once you kill the Servant, haul arse to a safer spot; heaps of Thralls and Cursed Thralls will spawn along with a Major Knight, the Hand of Crota.
Where is The Heart of Crota?
From the Rocketyards, head into the Refinery - just as if you were on the Devil's Lair strike. The Servant of the Heart is praying by a Seeder just before you reach the area with the wave battle in the strike.
Killing him will spawn a number of Thralls and The Heart of Crota, a major wizard. Retreat back towards the Rocketyards if you need a breather. When he's dead, at least four acolytes will rush you, so watch out for that if you're close by. Be wary of the Wizard's poison gas attacks, too.
Where are The Eyes of Crota?
For this one you need to search the Terrestrial Complex, which is where you complete the last main story mission on Earth in the core game. You can get here through Skywatch or via The Forgotten Shore. You want the area where the huge antenna array comes out of the ground. A Servant of the Eyes is kneeling with a sword in front of an urn. Kill him, then interact with the urn.
The wrath of the Hive descends in the form of a Tomb ship; its constant projectiles make the battle a little tricky. Knight Servants of the Eyes and Thralls will spawn non-stop until you take out all three Eyes; they're Major Acolytes, but not much of a challenge. The toughest thing about them is the way they run away and hide.
Once all three of Crota's minions are dead, return to Eris for your rewards and to begin the next step of the quest.