I just now realized that the cursed willow tree enemies aren't dark type, which I thought was strange since the progression to obtain them is very similar to corrupted demons, which ARE dark type.
Cursed willow trees and corrupted demons are both the third evolution of their enemy type, both unlocked in the same quest bundle, and both have similarly high kill requirements to complete their unlock quests. The cursed willows even have a color scheme that makes them look like the other dark type enemy evolutions.
With their current soul output and no dark buff, it's basically not worth the effort required to unlock them. You'd never have a reason to kill 25,000 cursed oak trees unless going for bestiary completion since most other dimensions give vastly more souls and coins than Mystic Valley by the time you've reached the point where you'd completing Combat Quests.


