Not really any main prerequisites, just that you must be level 40 and by the final quest, level 80. This is an endgame class so you should have probably met that threshold by now.

Someone asked for this I don't remember who, felt like this was mostly self explanatory based on the quest description.

• "Body, Soul and Domination"
  • Join /underrealm and defeat Fear till he drops 13x of his bones. May have to complete quests beforehand, it is a short map.
  • Head to /brainmeat and kill the brain matter for his drops.
  • Travel to /titanattack and defeat the Titanic Doomknight.
  • Defeat the Doomknight Armour in /valleyofdoom for his plating.

Requires Level 50.

• "Of the Same Cloak"
  • Join /necrodungeon and defeat the 4th Head of the 5 Headed Dracolich.
  • Join /lumafortress and defeat Corrupted Luma till she drops the Doom Worshippers Blade of Doom.
  • Head to /innershadows and defeat Krahen for the Empress' Shadowcloak.
  • Get the Cybernetic Doom Blade from /techfortress from Mecha Vortrix.
  • Get Asherion Armour from /stonewooddeep from Sir Kut.

Requires Level 60.

• "Refracted Light"
  • Head to /shadowoff and defeat the Shadowflame Paladin for their spikes.
  • Go to /fiendshard and defeat the Paladin Fiend for their horns.
  • Travel to /legionarena and beat the Dark Legion Paladin.
  • Defeat General Goldhammer at /noxustower for their hammer chips.
  • Grab shimmering tentacle from /chaoslab from Chaos Artix.

Requires Level 70.
Requires Rank 10 Evil Reputation (please read this guide for advice).
If I recall correctly these are all low drop rates.

• "Life Carve"
  • Join /wanders and defeat Sek-Duat for Trace of Light.
  • Head to /lightguardwar and defeat Extreme Noxus for Trace of Dark.
  • Go to /eternalchaos and defeat Bandit Drakath for Trace of Wind.
  • In /quibblehunt kill the Entropy Dragon for Trace of Earth.
  • Head to /crashsite and defeat Protosartorium for his Trace of Energy.
  • Go to /northlands and defeat Aisha's Drake for Trace of Ice.
  • For Trace of Water, go /deepchaos and slay kathool.
  • Head to /drakonnan and defeat Ultra Drakonnan for Trace of Water.
  • Go /battlefowl and defeat the Zeuster Projection for Trace of Bacon.

• "Soul Fracture"
  • Defeat alteon at /ultraalteon.
  • Defeat dehtrix at /ebondungeon.
  • Defeat sepulchuroth at /shadowstrike.
  • Defeat drakath at /ultradrakath.
  • Defeat gravelyn at /ebilcorphq.
  • Defeat fragment of doom at /shadowvoid.

Requires Level 80.
Most of these quests will require specific strategies or comps, I may go in more detail in the future.

• "Doom Spikes"
  • Defeat Xyfrag x5 at /voidxyfrag.
    • Get an LR to taunt(use scroll of enrage) the BLEEEEEEEECH, or do it yourself if you have LR.
    • Requires some people to help make this go faster.
  • Defeat Nerfkitten x3 at /voidnerfkitten.
    • Get an LR to taunt(use scroll of enrage) the Meow, or do it yourself if you have LR.
    • At a certain point if you cannot kill it fast enough, switch to Shaman, the 5 skill causes the enemy to stop dodging.
  • Defeat Doomkitten x20 at /doomkitten.
    • Use a class that doesn't crit or crits very unoften. For example like BB or DoT.
  • Defeat Voice of the Sea x10 at /seavoice.
    • Use vigils, buy a bunch of them from the npc there. Alternatively you can also buy them from /kathooldepths if you haven't completed the questline yet.
    • Whenever the enemy says "You cannot resist", you will have to use your vigil or you will die.
    • Alternatively, if you have chronos or someone can help you with chronos, you can nuke it as this boss doesn't have a shield.
  • Defeat Deadly Duo at /infernalarena.
    • It is a solo fight so you will not get help, you may need to do the previous fights.
    • Use archpaladin to reduce his outgoing damage and also reducing his lifesteal.
    • Alternatively, use scrolls of decay to stop him from healing.

• "Necrotic Blade"
  • The Shadow Reaper of Doom can be created from merging x300 Mirror Realm Tokens and 1x Undead Paladin Token.
    • Complete the [/mirrorportal] quest "Good, Evil, and Chaos Battle" for an easier time at getting mirror tokens.
    • Beat up Undead artix at /overworld till he drops his token, it is a 0.5% drop. Good luck lmao.
  • Read Sepulchure's Helm guide.
  • Read ArchDoomKnight guide.
  • Merge this at /terminatemple.
  • Merge this at /doompirate.
    • Farming the piece of Eight is difficult if you do not have LR.
    • It is recommended to run Archpaladin otherwise, or some friends if you still need help.

Do not be afraid by the big funni number, they give around 2k-5k each kill.

• "Unleashed Doom"
  • Defeat the inquisitors at /citadel.
  • Defeat the robots at /deltavcore.
  • Defeat the dragons at /etherwardes.
  • Defeat Chaos Vordred at /necrocavern.

You should now be able to obtain the class from the shop.