The attached realm cannot be attacked by flyers or swimmers. Giants defending this realm are immune to offensive magical items.
Copyrights:
TSR
Wizards of the Coast
This holding is a Forgotten Realms holding and can only be attached to Forgotten Realms realms, or realms that allow any holding to be attached.
This is a great holding, because there are several realms that it can be attached to that already limit the attackers to flyers and/or swimmers, and by attaching the Mist Caves, it effectively makes it almost impossible to attack the attached realm without magical aid. Some of these realms include Kozakura [FR-13], and the Coral Kingdom [4th-10].
This restriction also extends to the champion's allies. The ability to give defending giants immunity to offensive magic items is more of a secondary power, but if you have any giants in your deck, it's just an added bonus to an already efficient holding.