Is a character proficient with light armor, such as a rogue, considered to be proficient with mithral breastplate? What about a character proficient with medium armor, such as a barbarian—is he considered proficient with mithral full plate armor?
The description of mithral on page 284 of the Dungeon Master’s Guide is less precise than it could be in defining how it interacts with armor proficiency rules. The simplest answer—and the one that the Sage expects most players and DMs use—is that mithral armor is treated as one category lighter for all purposes, including proficiency. This isn’t exactly what the Dungeon Master’s Guide says, but it’s a reasonable interpretation of the intent of the rule (and it’s supported by a number of precedents, including the descriptions of various specific mithral armors described on page 220 of the Dungeon Master’s Guide and a variety of NPC stat blocks).
Thus, a ranger or rogue could wear a mithral breastplate without suffering a nonproficiency penalty (since it’s treated as light armor), and each could use any ability dependent on wearing light or no armor (such as evasion or the ranger’s combat style). A barbarian could wear mithral full plate armor without suffering a nonproficiency penalty (since it’s treated as medium armor), and he could use any ability dependent on wearing medium or lighter armor (such as fast movement).
The same would be true of any other special material that uses the same or similar language as mithral (such as darkleaf, on page 120 of the EBERRON Campaign Setting).
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