Dragons attacking this player's formation can attack only the attached realm; else they cannot attack.
Copyrights:
TSR
Wizards of the Coast
This holding is an AD&D holding and can only be attached to AD&D realms, or realms that allow any holding to be attached. This is quite the risky holding to include in your deck. The power only limits cards designated as dragons, and if your opponent(s) have none in their deck, it will be worthless.
However, if your opponent(s) have dragons, it can be quite devastating. This holding forces all dragons (champions and allies) to attack only the attached realm when attacking this player's formation. Exploit this by attaching it to a realm that puts further limits on the attacker.
A good choice is The Lost City [RR-16], because now the only dragons that can attack are non-flying, non-swimming AD&D dragons. An even more effective approach would be to have it attached to any realm in your formation except Border Garrison [DU-31], because now dragons could not attack any of your realms until Border Garrison is razed or discarded.
The same effect can be had by attaching the Dragon's Horde to Council Aerie [AR-86], which prevent dragons from attacking it. A dragon deck must have a good way to defeat this holding if it wishes to attack.
