Unique Charms have several special properties not found on other charms.

Only one of each kind may be held by a character at a time. This means that if you find a second one you will have to drop the first to pick it up. Simply stowing your charm in your stash will not work. If you want to save both charms, you'll need two characters.

In Single Player the only one that can drop is Gheed's Fortune.

Annihilus

This small charm can only be dropped by Über Diablo. It is only available on the Realms. It was added to the game in v1.10 and could not be found prior to that patch. It uses the graphic of Mephisto's Soulstone.

Dropped by: Only by Uber Diablo.

+1 To All Skills
+10-20 To All Attributes (varies)
All Resistances +10-20 (varies)
+5-10% To Experience Gained (varies)
Required Level: 70

Hellfire Torch

This medium charm was added in the v1.11 patch. It an only be found during the special "Pandemonium Event" quest. Firestorm is Diablo's Special ground fire attack. The random character class skills are NOT based on the class played. Good luck getting ones that will benefit your character.

Dropped by: Pandemonium Diablo.

25% Chance To Cast level 10 Firestorm On Striking
+3 to random character class skills (varies)
+10-20 To All Attributes (varies)
All Resistances +10-20 (varies)
+8 to light radius
Level 30 Hydra (10 charges)
Required Level: 75

Gheed's Fortune

This grand charm gives a substantial bonus to gold finding, magic finding, and even saves money in town. It can only be found in v1.10 and later. Gheed's Fortune spawns with one of the three Grand charm graphics chosen at random.

Dropped by: Any monster of a sufficient level. (Anything in hell, and some in late nightmare.)

80-160% Extra Gold From Monsters (varies)
Reduces All Vendor Prices 10-15% (varies)
20-40% Better Chance of Getting Magic Items (varies)
Required Level: 62