Similar to the warlock green fire quest, while bored at work, I came up with an idea for a death knight quest.
Added as a random drop at X, a soul burned sword can be looted, sending the death knight to speak with Mograine. He explains in northrend, the forge of souls in Ice crown citadel was one of two of such forges. The first forge was shut down long ago, but this blade appears to have been forged recently, past the destruction of the one in icecrown.
You are sent to northrend to investigate, but the blade leads you to a small tavern in storm peaks built into the mountain. Talk to the inn keeper, who will reluctantly give you the key to the back room in the basement. You enter a forge of souls like dungeon, where you have to traverse strange traps using your death knight abilities. Platforms that can only get to by death gripping the crystal, which deflects it and pulls you up instead. Fire traps you need to icebound fortitude to get through, etc.
You enter the final room, which will consist of 3 bosses. The first comes out, which is the room guardian. A purple marrowgar, who you simply need to move from his attacks, will of the emperor style. Outdps and surviv him, and the door opens. You approach the soul forge, and a Draenei paladin enters the room, telling you the power to bend souls, even with the purest intentions, is a road to destruction, and you fight him. Essentially like dueling a paladin. The final boss is similar to fighting the forge of souls boss in the icc 5 man, which is a long fight using abilities you need to use the correct ability to counter with. A life drain that needs to be counter life drained with death strike, attacks to be interrupted, etc.
Your reward is the replacement of a large amount of your unholy type spells to be turned into soul spells.
Soul summoning: Army of the Dead and Raise Dead will summon souls (lost of lorderon style)
Lost Shield: Your bone shield will appear to be similar to Soul Burn for Aff locks.
Icy Soul/Corrupted Soul: Frost Fever and Blood Plague will appear as small, untargetable souls attacking all over the targets to simulate dot ticking.
Soul Swap: Pestilince trades the souls of targets, appearing like soul drain effects.
Bloody Souls: Blood worms are tiny little souls that function the same as blood worms.
Ancient spirit: Gargoyle and upgraded unholy pet is now a pandaren spirit model.
Soul Burst: Blood Boil appears like the soul reaper effect proc.
Soul strike: death strike's healing effect looks like a quick Soul Drain before quickly fading
Soul barrier: anti magic shell is purple.
Death grip/gorefiends grasp: small souls pick up your target and bring them to you quickly.
To get a visual idea, anything you summon with the exception of the MoP style soul replacing gargoyle and infused zombie, will appear like the Lost of Lorderon
model ghost, same as the souls inside frostmourne in the LK HM fight. Anything not summoned, like Death Grip, or Pestilince, will appear like the Soul Drain effect, but with the soul reaper proc color.
I can't post pictures yet so that's the best I can do in explaination.
What do you think? Do you like the idea of unholy type spells being replaced with soul like spells? Maybe an option to go with an unholy/soul combo rather than frost/soul combo? What else would the green fire dk equivalent be other than soul like effects?