I don't know of any RPG before it that used Khajitts or Argonians.
Or had a world of that size and scope. Or that treated fatigue the way Arena does.
Or changing seasons or even a calendar with holidays, like in Arena.
And wasn't it the first one to use first-person, real-time combat?
Ultima Underworld was designed before Arena, if I recall correctly and had real-time combat.
(And I know there's one or two games before Ultima Underworld that had first person real-time combat as well)
Regarding Argonians, Arena didn't had much lore before Daggerfall came in. The Argonians in Arena were merely non savage lizardmen with their own territory. Lizardmen have been in many other RPGs before Arena. Again, Ultima Underworld had a community of them as well, and they had their own language. (A nice quest was to learn their language actually, very cool.
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Regarding world size, Final Frontier II: Elite has the whole of space, the whole galaxy ,with real-time distances between each planets. That makes it larger than Arena but, of course, not as detailed. That"s something I give to Arena. The huge, randomized world, but still detailed enough.
Changing SEasons and calendar with Holidays, I *think* the Ultima series had this already. (Not the first person titles, but the "over the top view" ones.)
But I think Amazon Queen was talking more about the storyline which was like a tale one woult tell his children to get them asleep. The story of a bad magician who banished the bearded king to a bad world and an hero travels around the world to find 8 pieces of a magical staff that destroys the bad mage. A world with human-sized cats and lizards and elves and dwarven weaponry.
In any case, I like Arena, but it"s not my favourite title. (Favourite title being Daggerfall)