Here's a Dark Souls 2 rare items and sets locations guide.
Most pieces come for monster drops, so it is advise to wear gold serpent ring+2, jester hat, prison tatters and use rusted coin to collect them.
Location: 3rd bonfire in Forest of Giants dropped by red npc phantom standing above the building where the bonfire is. He'll drop down when you get close enough, so be careful of his plunging attack. He will only appear once per play through and he drops each piece separately(he can also drop a broadsword+5). You might want to use bonfire ascetic to get all the pieces.
Location: Dropped by the two npc monsters that accompany the flexible sentry boss in no-mans whaft in NG+. Its suggested you summon lucatiel so she can distract the boss and one of the npcs while you take out the other. You can farm for the pieces by killing the npcs and then homeward bone/die to respawn them. They can also drop malformed claws and shadow daggers.
Mad Warrior Set
Location: Red phantom in belfry sol. This npc can spawn around the corner to the left after entering belfry sol indefinitely, albeit rare (1/10 times). You need to be in the bell keeper covenant for him to spawn. He can also drop a rare berserker blade katana.
Location: Red phantom located in Aldia's keep. The phantom will be behind the first painting on the left of the door if your coming from the second bonfire. This phantom also spawns once per play through and usually drops one piece at time so bonfire ascetics are helpful.
Aurous Set Non-Transparent
Location: Red Phantom on the ship of no-man's wharf. He spawns once per play through and drops one piece at a time so you need to use bonfire ascetic if you want all the pieces. He can also drop twin stoneblade.
Moon Butterfly Set
Location: Sold by Maughlin in Majula when you use bonfire ascetics on the bonfire in majula 2 times for all the pieces but the helmet), 3 times for helm (Need to have upgraded his stock by purchasing his stuff once for ~3000, then another ~12000 a second time to do this). The moon butterfly shield will be in the chest up the ladders at as well once 2 ascetics. (Don't know why but it's supposed to be available at NG++ which I was when I tried to buy it without using ascetics. Once I burned the ascetics they were available.)
Black Witch Domino Mask
Location: Dropped by red phantoms equipped with avelyn one the roof in belfry sol (there are two of these guys in NG+). These guys spawn once per play through. The mask decreases your hp but increases your magic damage.
Illusionary Ring of the Exalted
Location: Automatically stored in your inventory if you manage to beat the game without resting at any bonfire. (You can light them though. If you die you spawn at the last bonfire lit, you may also use primal bonfire to teleport back to majula)
Illusionary Ring of the Exalted
Location: Automatically stored in your inventory if you manage to beat the game without dying even once. It is suggested to use small soapstones to restock spells/health/etc but I've read somewhere that dying as a phantom might count as a death, while others say it doesnt count, so I'm not sure. Note: Don't die at all.
Pickaxe (Extremely Rare)
Location: Outside the first bonfire in brightstone cave, a smaller pink pig may spawn. This pig is hostile, and will follow you around trying to attack you. You need to escort the pig (making sure it doesnt die) and lead it to mushrooms located in the building after the bridge with crystals around it. It helps to clear out all the mobs in the path before leading the pig. The AI of the pig is incredibly stupid so take extra caution it doesnt run off any cliffs and die. You will be reward with the pickaxe once for completing this task. The pig supposedly has a 1/1000 chance spawn rate.
Thanks to aznkid for submitting the guide. If you have any other Dark Souls 2 rare items / sets locations hints, tips, secrets, or any other comments, please post them below. Thanks for your contributions and for visiting the site.