This multi part quest takes place on the facet of Tartarus. It is not easy but if you keep your wits about you it can be soloed. This one is also good for a group.
Begin by talking to the Morpheus the Monk on the island of Westernesse Tartarus. A Moongate and Recall Runes will become your friend.
1) He will ask you to find and slay the four horsemen. He needs you to bring him the Helms of the horsemen as proof of your victory. He will give you four books with directions to each of the horsemen. Make sure you keep those books in your pack or your search will be in vain.
2) Each of the four horsemen wander inside a Tartarus dungeons and are seldom found in the same location and never found together. In addition to their Helms they also carry a Special Item. If you have all four of the Special Items they can be combined to make one Valuable Artifact.
3) When you return to the Quest Giver with the Helms and the Special items he will help you find the Orb of Artifacts which will convert your Special Items into the valuable Artifact.
4) There is a possibility to obtain four complete sets of Armor Artifacts enhanced to the Fortification level. If you get two of the same piece then a Tinker can Enhance them to the Highest level (Invunerable). At that point you can add or remove Ancient Jewels to modify properties or you can get a Spellcrafter to help.