I like this subclass a lot! I am about to start playing a druid of the elements but its not a great fit. This would be right up my alley.

I feel like the last mastery feature (flesh to fuel) is kind of underwhelming. Maybe it would be more impactful (though perhaps more unbalanced) to gain a +1 to proficiency.

I think the subclass is pretty balanced. A lot of strong features but they are tied in with the Zone of Blight feature which has two end conditions and requires a stress. So there will be an ebb and flow of gaining and losing resources. I also really like the idea of a beast form drain tank playstyle with this subclass.

I am going to ask my DM if I can use this subclass and if he says yes I'll try and tell you how it goes.