Get that steed for Lash!

Here's a wee walkthrough:

Head to the west wing of Dire Maul and proceed to clear the pylons and open the way to Immol’thar. Kill the big dog then med up and activate your jar that starts the quest. The Imp will set up a wheel, candle and bell. Do not start this event without at least 15 soulshards in your inventory as you will need to use many during the battle.

During the battle, the candle will deal damage to all enemies within the ring, the wheel will reduce damage taken by your group inside the ring and the bell will heal people inside the ring constantly. You, as the warlock, must keep all 3 of these working. When you see one stop working you can fix it by moving to it and right clicking it which will also expend a soulshard. If you run out of shards it is game over.

You must keep the things working while waves of demons come. They are of 2 varieties. Non-elite cannon fodder imps and elite felguard type demons. The imps are not much to worry about unless they are interrupting you trying to fix one of the parts of the ring so just keep their aggro off you and the candle will take care of them. Aggresively take out the elite felguard mobs as they spawn and use banish and/or enslave as necessary to keep from getting overwhelmed. Having 2 warlocks along for this part is not a bad idea and they both get credit.

Once you’ve been at it for awhile all the demons will disappear (when banish wears off those demons will disappear and so will enslaved demons when you cancel enslave). At this point you should med up and then summon the dreadsteed. The steed is a 61 elite that hits pretty hard and has a very powerful knockback attack. It is joined by a 62 elite dreadlord (the one you’re stealing the mount from) who will sleep members of your group but doesnt hit especially hard.

Try to stay within the circle during this fight as its benefits will still apply. Once the dreadsteed dies, a spirit will pop up that you can speak to. Speaking to it will complete your quest and teach you to summon the dreadsteed.