The one and only Myztero
Good luck
What other Purples do you have maxed?
Without knowing anything else I’d say Kage (unless you have the costume for Dom, then I’d say her).
Edit: you DO have the costume. I’d go with Domitia.
Sorry for the late response been really busy.
So you have (in 5*) two awesome purple tanks & a very fast sniper/DoT.
I would PROBABLY go with Kage or Clarissa personally.
Kage will add a second Very Fast sniper to pair up with Victor. Clarissa will add a second Very Fast DoT.
I personally wouldn’t go Myztero at this point, as he’s pretty new & very niche.
Dom does get some consideration from me cause you have her costume there which a) is awesome and b) boosts her stats up to being comparable/ better than Kage & Clarissa.
So reading that I realise I don’t actually give concrete / clear-cut advice 
I would take Kage & Clarissa to 3-70 & experiment a little bit with the two. Work out which fits your play style better. Then go with that one.
This is my 1st post , so I should have been more clear of what purpose the next hero would serve. My defence is pretty much set. My next dark hero would will be on my mono attack team for war. Just wanted some good feedback from more experienced players. Thanks for advice.
I would do Myz just for the fact of the rarity and something fun to learn. Perhaps look for YouTube videos to see the best way to use him? You are stacked with Purple, I’d go with the wild card.
The cynic in me thinks this was just a veiled attempt to show us you have Myztero…
But I’m happy to be wrong 
If you mono, I’d go with Clarissa for her elemental link.
And welcome to the forum. 
You’re really wrong.
Thanks for all the positive feedbacks.
Watch YouTube all the time, actually taught me how to play the game. 
Thank you. Hope I can give good advice some day.