However, I could tweak it to where the MAX is depleted like a Shard of Void...but I have a feeling people would yell OP before anything else.
Since the wording leaves the meaning a little ambiguous, its just a matter of whether you state it in the notes or not I think.
If max HP drain is included, the drain at each tier could be lowered to compensate. It also would make it more random, which fits
better.
E.g. instead of:
1-3; 4-6; 6-9 ...
it could be
0-2; 1-5; 2-8 ...
Thematically, loss of max hp fits the ability "degradation" very well, which is a plus.
Draining max hp in addition to normal damage definitely makes it more powerful, but it also makes it fill a unique niche since there are currently no cards that can drain max hp from creatures. Max hp drain is very useful since it prevents the targets owner from simply healing it to compensate for the damage.
Either way, its still a cool damage buff. More powerful than poison, but a little less consistent.
On a similar note to voodoo dolls, what happens if the target gets shot at the opponent by a catapult? Would the enemy player get afflicted by degradation?