An adult dragon is an experienced, intelligent being whose presence can influence the entire surrounding territory. A lair in a mountain fortress, extensive underground chambers, or a remote ruin is more than a resting place: it may hold supplies, keepsakes, valuables, and records of old agreements. Its origins and lineage may reach back to events local people know only through legends. Its relationship with the territory is usually far longer than any individual ruler's lifetime.
It generally makes its own decisions and can plan far into the future. It may maintain servants, allies, or dependent settlements without sharing an ordinary home with other dragons. It seeks security, influence, and things it values; knowledge or a fulfilled oath may be as valuable to it as gold. Its nature varies. An adult dragon may be a destructive tyrant, a steadfast guardian of an agreement, or a being that regards short-lived human disputes as disturbances of its peace.
Once per battle, Dragon Breath can strike up to two targets. Ancient Scales reduces the first instance of physical damage and the first instance of magical damage each round. If the party discovers and actually exploits its particular weakness, this reduction stops applying and both defenses drop by two. Investigating the weakness is therefore an important part of the expedition; the particular story determines its nature, and clues must support it. An agreement, repairing a broken promise, or an offer that removes the real cause of the dragon's hostility may be equally significant.