You taint a living creature's blood with a hot, corrosive infusion, dealing 1d8 points of acid damage and 1d8 points of fire damage per round. The subject can attempt a Fortitude save each round to negate the damage, but a successful save does not prevent damage in future rounds. Searing pain limits the subject to a single move action in any round when it fails its Fortitude save.
Burning blood does not affect creatures of the construct, elemental, ooze, plant, or undead types.
So... they're covered in thorns, which burst as area-of-effect spells whenever it takes a hit. The "closest" to the skeleton takes the burning blood effect. Let's drop that bit in favor of whoever actually triggered the spine burst.
Sounds like it happens whenever the skeleton hits someone too.
Also: do we want to keep the "recharging" bit? Seems like they only get one shot at the spell as written.
I like the idea of treating the spikes as armor spikes for attack purposes.
Rather than dealing with the messy recharge vs. 1/day bit, why don't we just simplify the burning blood to death throes?