So like this then: When this champion is targeted by a Mutate ability, this champion gains Fear 2. If it already has Fear, it gains Frightful Blows instead. If it already has Frightful Blows, it gains Dread instead and this ability is removed.
In other words, a Champion with Mutation: X would REQUIRE a Mutate ability/effect from another rune for them to actually gain any abilities. And Mutate cannot be used until you farm some Charges on that champion. Seems pretty hard to cost and balance, but I'll think about this for a bit.
Keep in mind that people don't like theme enablers, and even less if two of them are Legendaries. But I agree on the earning charges abilities part. It's cost is complety flawed because of the formula.
I think people THINK they don't like theme enablers more than they ACTUALLY dislike it, but I guess it depends on the definition of enabler. In this case, the theme simply wouldn't function MECHANICALLY without those runes, so it's less of an enabler and more of a requirement.
For what it is worth, if you are going to move forward on this, I would give The Doctor Resurrect in place of Rabid to make this theme better. That is just IMO.