Played around a bit and I'd like to add that the multi target debuffs are too op for their cost.
My thoughts are adjust them with a combination some of the following:

Single target, reduce rebuff duration, increased cost, add moderate physical/magical damage or miasma cost. Maybe let It have one multi target debuffer, but that one can be expensive.
Thank you.

This is cool and creative. My thoughts:

I like scythes. I like dark.

I don't think the archer sprite suits the class, maybe hexer/reaper/assassin.

I haven't used it yet, these are my First impressions and will play more. Abilities seem well thought out. Taxing, but impacting. However as a starting class it's too expensive and probably op. You could plop it down midgame somewhere,

Maybe add/change a skill so it's not fully MP reliant? Next to the steep costs, miasma armor needs to be active for some, I like the idea of sacrificing defence, but it also costs an arm.

Or, keep the costs and have some abilities remove a tick of miasma, and maybe 2 for stronger attacks, instead of the whole thing. And then probably nerf the initial buff from 3 to 2 or one turns, and the skill from 5 to 3 turns.

Just my thoughts. Thank you.