Once you you have your Great Dragon armored and capable of attacking, you can start attacking level 6 and higher forests. A chest will then drop, which has a chance of rewarding you with up to 3 toxic dragon crests.
You must already occupy your maximum number of wilds, since no crests will drop if the wild is conquered. Similar to other marches for crests, the attack must be accompanied by your Great Dragon.
The marches will earn the player up to 3 crests daily, with this amount occasionally increased with events.