
avr |
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It's an old, old rule. Pre-AD&D. Probably it was put in once upon a time because someone writing a scenario wanted all the zombies or whatever stopped by a wall of fire & didn't want to upgrade the spell as a whole. It then just survived the editions because no one bothered changing it.
It's certainly possible to argue the fire burns, fire purifies, undead are impure, but that runs into the 'what about fireball?' argument. I think the above is more likely.
Edit: the way PF stacks mutipliers, double damage and +50% damage makes for x2.5 damage.

lemeres |
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It's an old, old rule. Pre-AD&D. Probably it was put in once upon a time because someone writing a scenario wanted all the zombies or whatever stopped by a wall of fire & didn't want to upgrade the spell as a whole. It then just survived the editions because no one bothered changing it.
I think I could bs it and use that logic in setting. you could say that the spell was developed by a wizard that was serving a kingdom besieged by a necromancer. So it was a spell designed to stop undead from the get go.
At the very least, this is what I would spout as a GM to get the player to just continue with the game. Maybe say that the player passed a DC 10 knowledge (arcane) check, and remembered something from their "history of common tactical spells" elective class.

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Wasn't it druids did but wizards didn't in AD&D. Than they just copy and pasted things together? Could be remembered wrong. Don't have books and on phone so now sure where to look up.
That's how I (perhaps wrongly) remember it, also. Druids lacked the undead turning power of clerics, so several of their spells, such as magic stone, flame blade and wall of fire, did double damage to undead. Wall of fire wasn't the only spell that worked differently when cast by a druid, IIRC, there was also plant growth, which had an extra option to increase harvest / yields, while the 'magic-user' version was basically a way to make crappy terrain.
I'm sure the extra damage vs. undead could be rationalized by the druid drawing upon the forces of nature to scourge the unnatural, or the power of life itself to destroy the dead. A druid version of hydraulic torrent that used a flask of holy water as an added material component and knocked down *and* damaged undead could be similar, in theme.