You know what would be REALLY amazing? If this had a similar effect to flood,
but it 'spreads' from one row/column/creature slot to the next (rather than taking up two whole rows with an upkeep factor), and unless the creature is immortalized (or burrowed, but that might not make sense), they get TOASTED. Then the lava 'cools' off from older areas, so a creature can be played on the previous spot instead.
As it is now,
it's just a rip-off of Eclipse/Nightfall. It's a nice idea, but as others have said, it gives Fire way too much offensive potential/defensive potential, when Fire is truly about the 'all-encompassing' offensive arts. Keep it as a permanent (Fire has too many spells...), but I say try working it a bit differently.