Wiki - Apocalypse
The Halloween Dungeon is in Khaldun Felucca.
Part 1 - Four Horsemen of the Apocalypse
Find Dupre waiting just outside the castle of his old friend, Lord British.
Collect four Seals from Dupre after defeating the four horsemen. They are the keys to reach the Grim Reaper on the second part of the quest.
The Four Horsemen Are:
Pestilence - White Horse
Famine - Black Horse
War - Red Horse
Death - Pale Horse
Part 2 - Facing The Grim Reaper
Find the Peerless Altar in the dungeon and place the four seals you have into the braziers.
Once all four seals are in place, a gate will open on the Altar. Step through and prepare for the final battle with the Grim Reaper. Defeat him and bring his Scythe back to Dupre to collect your reward.
If you are partied then it's possible to have your friends help you with the Grim Reaper, but be aware that only one Scythe will drop so multiple runs may be needed to allow for everyone finishing the quest in these circumstances.
You will receive a reward bag from Dupre for completing the quest. The bag will give you 1 random reward from the Apocalypse Rewards Display Area. Large Rewards are displayed upstairs. Please note, a few of the rewards in this display (The Vats) drop off of the Samhain Champion Boss, and the Grim Reaper's Soul Shearer and Skeleton Sitting Area Addon Deed both are special drops from the Grim Reaper.