wow wow!! this idea is awesome. You could do a serie, a new mark for each mark.
I think pendulums shouldn't work for this mark (a handicap) but make it powerful (as it is). The cost is fine as 0, just replace your mark.
And if you agree to do the rest of marks, entropy one could get a random mark.
There is an equivalent card for every element.
I just chose to use fire as a random example, sort of cause "Pyromancy" sounds good.
It may have to cost something, but the main balance points are rarity to a degree, play limit, mark replacement, hand limit, and randomness. I'm not sure, it may be enough.
EDIT: Another balance point is, of course, that every element gets an equivalent.
It would be a great extra card to stick in a deck, but not really designed to be central to your strategy.