I think it's mostly fine. Fire cauterizes wounds, although it comes at the price of a lot of pain (and burns too) So perhaps it would be better to replace non-burn conditions with a burn for each one that was removed, rather than taking direct damage? Maybe give the choice of how many conditions are replaced with burns?
I don't like replacing with Burn Conditions at all. Not all conditions are made equal, which is the point of removal costs to begin with. I'm uneasy actually with basing the damage off of half the removal cost, because that reduces the difference between conditions. I would almost like for the damage to equal the removal cost, but that is a really steep price to pay. You'll regain a loss of mana to pay for removal costs, but regenerating damage is not so easy. Thus, I will accept that the damage is equal to half the removal cost, but replacing with Burn Conditions could be a detriment, an improvement, or a simple trade and I don't like it.
Well, the average damage done by a burn is 3, and removal costs are typically 2 or 4, so in general this proposal would make the spell even more damaging. On the other hand, burns aren't too difficult to remove if you come prepared. As Zuberi states, it is probably a good idea to tie damage to removal costs. Originally, I had intended for the damage to equal the removal cost, for simplicity. Then I looked at the removal costs for the conditions one would typically remove, and decided that it would be too painful that way.
As to the Holy vs Dark issue, to me it seems that the pain and the price is coming from the Fire portion of the spell. I don't see what Dark is contributing to it at all. It is unnecessary, makes no thematic sense, and breaks down the boundaries between the Schools of magic that make them distinct. Making it Fire + Holy would explain all effects of the spell, be thematic, and remain within the parameters of existing canon. If instead, you want to give condition removal to a different school and break from tradition, then you could simply make it a single school spell.
Perhaps it might help if I explain the inspiration for this spell. I was originally thinking of making a "cauterize" spell to stop bleeding. Unfortunately, bleed is a pretty specific condition, and such a spell would likely never see its way into a spellbook due to its specificity (besides which bleed is already pretty easy to remove). So I decided to keep the idea of purging conditions using fire, but broadened the scope of the flames so they could burn almost any condition out of the target. Clearly, this goes beyond the scope of normal flames, so some other magical component is necessary.
You suggest holy as a second school; holy was actually the first school that I considered ("Cleansing Flames"), but ultimately decided that making it dark was better for a couple of reasons:
1. The holy school already has numerous ways to deal with conditions. Though this spell is versatile, I can't really see any holy mage choosing it over other, less painful options.
2. The theme of sacrificing life or suffering damage in exchange for some benefit is thoroughly dark, as Imaginator notes.
3. Currently, I feel that the holy school has too much of a monopoly on condition removal, a broad category of mechanics. This will change somewhat with the release of cards like Purge Mind (I think that's what it's called), but we need some alternatives.
While I believe that it is necessary to maintain boundaries between different schools of magic, I don't believe that the category of "mechanics that remove status conditions" is something that ought to be restricted to one school. The Holy school should definitely have the simplest, most straightforward spells in this school (in much the same way that the arcane school has the most straightforward antimagic spells), but I think there is still scope for other schools to have more restrictive, conditional alternatives. In this case, Hellforged Cautery gives the warlock a way to burn conditions out of himself or others, which is certainly more painful than magicking them out.
Thematically, the flames of hell will burn out any weakness. Does this rationale make sense?
