Instead of coming up with potentially boring Arena ideas myself, I decided that the combatants should come up with arenas themselves. Just give me the general setting for an Arena then a few thing you would want to see or would find cool to add to said Arena.
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The Sacred Grounds An Eclipse ruin. There are portals that lead across the multiverse and objects phasing in and out of reality. There is a massive statue featuring a symbol of an eclipse. Sometimes cultists wander in on a pilgrimage and will attack.