["Hollow Mountains"] (Non-Quest, Non-Story.)

Hollow Mountains appear fairly late in the games, not long before the end of the first age. To describe them in a single word I need only reference Tolkien: Moria.

These immense mixes of cave, mine, city, temple, and dungeon will be filled with boobie traps, monsters, and riddles enough to challenge any pantheon, but also will be the last great treasure troves of divine gear, primal icons, and other items suitable for deities.

If a pantheon can fully clear a "hollow mountain" it will make the perfect place for their worshippers to found their capitol city, but beware, you must be exceedingly careful that you have fully cleared it, or it might suffer Moria's fate...