Notes on purity below
5 cost, 10 morale
At the end of each turn, Unstable Hydra gains +1/+1 for each point of damage it has taken that turn, then gets +5/-5
As Veroria fell into chaos and destruction, Aleta's forsaken Hydras emerged into the corruption of the wastes
For whatever reason, this contest reminded me of Verorian Hydra. Given the current goings-on in Reisch, I decided to come up with an updated version of the card reflecting the expanding corruption of the Demon Wastes. Initially I just had it mirror the original Hydra (being a 6 cost, 12 morale beast) but as I worked on it I decided to change up some of the numbers. I focused on the number 5, because it seemed reasonable. The Hydra will, barring outside buffs, last 5 turns (to reflect how unstable it is). It has 5 power. It costs 5 resources, and has 10 morale (2x5). Given this, it probably shouldn't be run in command unless you're running Hermit, Aleta, Daode or something. Alternatively, the numbers could be changed to 4 (which is associated with death in certain cultures), though in that case I would up the starting power. The story being told here is that as the "magical fallout/waste products/radiation/screw you, EPA" affects the Hydras they become more powerful but also easier to kill.
In terms of purity, I would ideally like for the card to be CoV OR Exiles, CoV OR Exiles (if that makes sense). That way, it could go in a 2 CoV, CoV/Exiles, or 2 Exiles deck. As is, I'm leaning towards either 1 CoV/1 Exiles or 2 CoV.
Originally, I just had the hydra lose 5 max health at the end instead of getting +5/-5, but I felt that this was more flavorful, as the corruption warping and destroying the hydra from within is still making it more powerful even if it isn't taking any damage. Otherwise a Hydra that got consistently avoided by the opponent (perhaps through stumble effects or by pulling back their characters) would eventually wither to being a 5/5. Which never feels any good and shouldn't be an easy option for counterplay.