Once you you have your Great Dragon armored and capable of attacking, you can start attacking level 6 and higher forests:available on new realms.
In hopes of getting Toxic Dragon crests as a random drop.
You must already occupy your maximum number of wilds: if your attack occupies the wild, you won't get a random drop. Each attack must be accompanied by your Great Dragon. The Great Dragon must heal before you can attack again, so this can take a long time, as you need 100 crests, and not every attack will drop a crest. There is hope, as some drops include more than one crest.