Here's the latest subclass that I've made. Any feedback would be greatly appreciated. 

https://www.dndbeyond.com/subclasses/627685-circle-of-devastation

i love it! i don't know a lot about druid circles, as i don't play druids much. and when i do, i almost always play spores, so i could be wrong, but it seems kinda weak, except for the level 14 ability seems op to me. the flavor is amazing though. its everything the spores should have been. definitely going to play this!

Quote from WarlockThatSlaysDragons >>

i love it! i don't know a lot about druid circles, as i don't play druids much. and when i do, i almost always play spores, so i could be wrong, but it seems kinda weak, except for the level 14 ability seems op to me. the flavor is amazing though. its everything the spores should have been. definitely going to play this!

 Thanks so much!

It's nice, but suffers one big problem: It's very situational.

Mostly: Empowered Health, Decay Restoration and Nature's Terror.

The first one is a scarce thing, and though it does occasionally happen, as a druid, you can get many spells to cure diseases anyway.

The second is nice, but not always will you enemies deal poison damage. If they don't, the ability becomes useless by nature.

The third is cool, maybe, but you don't fight that many plants and beasts at these tiers of play. The highest plant/beast CR is 9 from basic rules or 12 from other sources. At level 14, you could probably easily take on these CR levels with ease, unless they come in mass numbers.

My suggestion: 

Change Empowered Health to a completely different feature that gives them resistance against poison.

Expanded Spell List: Change the spells to spells with more destructive powers, as that's the theme if I understand correctly. The classic Fireball, for example, is a great spell to add there, even if nobody actually likes it. Destructive Wave? Sounds pretty fitting to me. Stinking Cloud, Ray of Enfeeblement and other spells you chose... they're not exactly destructive.

6th level ability is great. Maybe, just maybe, change it to one use per long rest. You should playtest it.

10th lvl: Again, it turned into a pretty defensive subclass, instead of a destructive one. I have an idea, but you can change it to something else. My idea is receiving a special spell slot that is automatically increased. Using that additional spell slot, you can cast any spell of below spell level 5 (possibly increased to 6 later, if you feel like it,). Once cast with that spell slot, the spell is automatically 2 levels higher than it's base form. It will reset on long rests. This spell slot must be used to deal damage though. Or maybe another restriction, because reading while writing this I understand it's broken as hell. It's just an idea but it might give you inspiration. The point is, I think you forgot to make the subclass as destructive as its name and description suggest.

14th: Again, you're not going to encounter many beasts and plants at these levels. Even if you do, you won't even need this ability to kill them. Look for something more destructive and less situational.

These are all only suggestions though, do as you see fit.

Quote from FireCat5 >>

It's nice, but suffers one big problem: It's very situational.

Mostly: Empowered Health, Decay Restoration and Nature's Terror.

The first one is a scarce thing, and though it does occasionally happen, as a druid, you can get many spells to cure diseases anyway.

The second is nice, but not always will you enemies deal poison damage. If they don't, the ability becomes useless by nature.

The third is cool, maybe, but you don't fight that many plants and beasts at these tiers of play. The highest plant/beast CR is 9 from basic rules or 12 from other sources. At level 14, you could probably easily take on these CR levels with ease, unless they come in mass numbers.

 

My suggestion: 

Change Empowered Health to a completely different feature that gives them resistance against poison.

Expanded Spell List: Change the spells to spells with more destructive powers, as that's the theme if I understand correctly. The classic Fireball, for example, is a great spell to add there, even if nobody actually likes it. Destructive Wave? Sounds pretty fitting to me. Stinking Cloud, Ray of Enfeeblement and other spells you chose... they're not exactly destructive.

6th level ability is great. Maybe, just maybe, change it to one use per long rest. You should playtest it.

10th lvl: Again, it turned into a pretty defensive subclass, instead of a destructive one. I have an idea, but you can change it to something else. My idea is receiving a special spell slot that is automatically increased. Using that additional spell slot, you can cast any spell of below spell level 5 (possibly increased to 6 later, if you feel like it,). Once cast with that spell slot, the spell is automatically 2 levels higher than it's base form. It will reset on long rests. This spell slot must be used to deal damage though. Or maybe another restriction, because reading while writing this I understand it's broken as hell. It's just an idea but it might give you inspiration. The point is, I think you forgot to make the subclass as destructive as its name and description suggest.

14th: Again, you're not going to encounter many beasts and plants at these levels. Even if you do, you won't even need this ability to kill them. Look for something more destructive and less situational.

 

These are all only suggestions though, do as you see fit.

 Thanks for the feedback!

Version 1.2: https://www.dndbeyond.com/subclasses/670039-circle-of-devastation

Changes:

Ray of enfeeblement replaced by inflict wounds.
Black tentacles replaced by blight.
Empowered Health replaced with Sickly Defense.
Decaying Restoration replaced with Destructive Overflow. 
Nature's Terror buffed to include humanoids.
Nature's Terror buffed to deal 3d10 damage. 

The subclass should be a lot more destructive now. Thanks a ton for the feedback, WarlockThatSlaysDragons and FireCat5!