Rules: Avoiding Twilight

Any dragon that has reached the old age category or higher can depart, become a guardian, or become a dracolich.
To depart or become a guardian, a dragon must consume at least 135,000 gp worth of treasure—or at least 90% of its hoard, if the dragon possesses treasure worth more than 150,000 gp. All the treasure must be consumed in the same day, and at least 120,000 gp of this treasure must have been part of the dragon’s hoard for at least 200 years.
After consuming the required amount of treasure, the dragon must find a dragon graveyard or suitable site to guard. Once it is at the graveyard or site, the dragon completes the process instantly simply by willing it to happen.
When a dragon departs, its body dies. When a dragon becomes a guardian, its body melds into the landscape. Once it has consumed the treasure, a dragon can delay departing or becoming a guardian for no more than 1 day per point of Charisma it has. If the dragon exceeds this time limit, the opportunity is lost, and the dragon cannot make another attempt to depart until it again consumes the required amount of treasure.