I wish Magma had more fleshed out lore, but here’s what I’ve got:
They were created when an island nation refused to give a sacrifice to their volcano for protection, and so the volcano god sent a lava monster, Magma, to punish them. Then, centuries into the future, archeologists find the temple of the volcano god, awaken and anger it, and then Magma is summoned again. (Then they fight a whole bunch of other gods and monsters, but I haven’t really been able to work out those details yet.)
The whole thing is very inspired by The Mummy (the one with Brendan Frazier), Godzilla, and the Pokémon Adventures manga.