Extreme Damage Resistance (5e Variant Rule)
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Extreme Damage Resistance
As a DM, have you ever had a player complain to you saying, "I have chainmail of fire protection, and I am a red dragonborn, why don't I have fire immunity or something"? Have you ever been making a homebrew race or monster and been like "Fire immunity is too strong, but I don't want to just use resistance because they are better at resisting fire than some common fiend"?
That is why I made extreme damage resistance. Extreme damage resistance is just double damage resistance or resistance applied twice, applying half damage and then applying half damage again, resulting in 25% damage taken. An example would be: an adult red dragon breathes fire at a red dragonborn fighter who has chainmail of fire protection. The dragon's breath attack does 60 fire damage, if it was resisted normally, the dragonborn would take 30 fire damage on a failed save or 15 fire damage on a successful save. With these rules, the dragonborn would take 15 fire damage on a failed save or 7 fire damage on a successful save (in line with 5e's "Round Down" philosophy).
While you can just use extreme damage resistance for when you happen to have two damage resistances and you want to calculate both, there are more possibilities for this rule. While a DM who wants to use this rule could attempt to incorporate into his campaign, I more expect it to be used for homebrew races, classes, and monsters.
To that end, I am adding an example feature to make it as easy as I can to add to a homebrew article.
Extreme <damage type> Resistance
You have extreme damage resistance to <damage type> damage.
Stacking Damage Resistance
This rule was made with the idea that damage resistance would only be stacked once or it would be used on another homebrew page. In theory however, you could continue stacking damage resistances in this way. A word of warning though, after a certain amount of stacked resistances, whoever is receiving the damage can become effectively immune to that damage (even if they are not technically immune), which is what this rule was trying to avoid.
Extreme Damage Vulnerability
This rule could be inverted for damage vulnerability but damage vulnerability is rare in 5e and there are very few magical effects that apply it. Something else to consider is that having anything take four times damage from a damage type is very powerful and very destructive.