Eris, Bane of Crota, has a new quest for you - protect Rasputin.
Destiny: The Dark Below guide - Siege of the Warmind
Destiny: The Dark Below picks up the threads of the story in the main game, too. In this second mission, we've got to prevent Crota's agents from finding Rasputin, the last warmind (probably). He's a bit of a dick, to be honest, but also he has power we don't want in their hands.
Siege of the Warmind is available at difficulty level 25 and above, and takes place on Earth. Solar weapons are a big help against the many shielded wizards you'll encounter.
You start in the Mothyards and need to head to the Forgotten Shore. It's finally time to enter Bunker RAS-2.
The Knights who normally guard this area are absent, replaced by ominous music. Head down once the doors open to face a knot of Hive. There's an acolyte and Knight ahead, but stay out of their view and pick off the swarms of Thralls first.
Push forward and turn left up some stairs. On level 28 and up, this area is ridiculously populated with Thralls. Keep picking them off as you slowly ascend the steps, only to meet a Major Knight. Kill him to open a door to the next area.
In this area, you'll face a wave battle. The arena is much easier to navigate than the last mission's, but it's still a tense, difficulty battle. First, you'll meet an Ultra Wizard. Just blast away at her for a bit, minding out for the Thralls around you, and eventually she'll bugger off outside the arena; you can see her through the glass.
Clear out any remaining enemies and then wait and watch. You should see a dark green spawn point appear in the air. The Wizard is going to do several of these, each harder than the last.
Each wave contains Knights, loads of Thralls and an increasing number of Wakers of Crota. These special Wizards can put an area of effect damage point down called a Vigil, which you'll then have to avoid. They also hit really hard.
Generally, I found it a good tactic to hang about near the control panel, out of view of the Wakers and Knights while I dealt with the Thralls, then to peak out and carefully pick off the bigger enemies.
You have to face multiple waves before the final battle, so in later rounds (there are more the higher the difficulty is), you'll face two, three or more Wakers. In this case, it's a good idea to start off by taking one Waker (or more!) down with a Super or Heavy weapon before retreating to your platform shelter - otherwise they can easily pin you down.
In later rounds on higher difficulty, you'll see more Knights, a couple of Major Knights, and many more cursed Thralls, too.
The last round is a battle against an ogre called Might of Crota. He's not hard if you can stay out of sight, but watch the sides of the room - melee Knights will charge you. On higher difficulties, this is very, very hard.
When you've won, return to Eris in the Tower for the next quest.