Instructions
Head to the second layer of BattleUnder, BattleUnder B, and speak with the paladin Yara, who you originally met in the first level. Yara will give you a new set of tasks and tell you about the locked chest and the keys it requires.
Kill all the Skeleton Warriors in BattleUnder and collect the six Glowing Wood Fragments. Return these to Yara to assemble the Key of Life. Open the chest with the Key of Life for your first reward.
Repeat the previous step to collect the Shining Crystal fragments and get the Key of Dreams. Repeat the previous step again for the Pitted Stone fragments for the Key of Thought.
Destroy the Undead Champion and reclaim the Key of the Void. Unlike the previous three keys, this one remains intact.
Return to Yara and open the chest one last time with the Key of the Void. Your reward this time may include the Deathknight Dagger, although you may end up with the Deathknight Staff, Blare or Spear.