"Oh no... we're here."
And she screams when a lightning bolt hits a tower less than three hundred metres from where they're standing.
"Very well," he says, and clears his throat. "Let me tell you about the Crossing. A long time ago, this place was a traveller's nightmare. 'Plains of lightning, plains of thunder, those who cross are torn asunder,'" he intones. "Or... so they used to say. Then a man named Bilghen appeared. Using machina, he built towers that served as lightning rods across the plains. Finally, travellers were able to cross in safety. Bilghen was building that tower over there..." he points, "when he was struck and killed by lightning.
"Bilghen was an Al Bhed. Our history books never mention him.
"And that, as they say, is that."
It tells the history of High Summoner Gandof in some detail, but the relevant passage might be this one:
Long ago, fiends known as Qactuars roamed the Djose Continent freely. They ran rampant, ravaging fields and tormenting travellers. Then Lord Gandof appeared. He scoured the continent for the Qactuars and drove them to these plains. Once he'd gathered the Qactuars together, Lord Gandof linked the sign of the fayth, whereupon the Qactuars turned to smoke and were absorbed by a stone. The Qactuars were thus sealed into stone, fated to face lightning day and night. Unless someone again links the fayth sign and releases the seal, they shall suffer within the stone for their deeds. Thus did Lord Gandof bring relief to peoples all over Spira. He believed that the battle with Sin was only one great part of a summoner's greater mission.
"I don't know if I'd say they terrorize people but they do attack. When fighting—" A particularly loud lightning strike hits a tower nearby and she yelps. After a couple of seconds she continues, shaking: "F-fighting c-cactuars you d-d-don't usually kill them, though, that'd be evil."
"… I'm a bit confused by everyone just ignoring the fact they're unlike other fiends and just going ahead and calling them fiends anyway? And – there are more just blooming in the desert, so – were they just all over before, and this guy rounded them up, and they don't care about the others?"
Well that's very nice of it! He would have felt worse if it seemed biological, since Rikku said cactuars leave bodies! (And also he probably would have stopped quite briefly after making it clear that you do not attack him, if it seemed like it were leaving a horrible mark.)
He stops short of trying to actually kill it, or do more than cursory damage with the alkali, and moves on to trying to disable its movement instead.
Nope, this is the introduction! The book goes on at length about the historic uses of elements and the various trials of alternatives, and also explains the influence of Sin on research (basically made it stop). It gives examples of elemental magic and elemental fiends and situations where elemental magic's called for and when it isn't, mentions enchantments and items that have elemental attunements...
It would be really nice if Sin could just go die in a corner.
Theo does not think it wise to spend magic into lightning protection, probably, because brief burns and a mostly-unexpected shocking pain is ultimately not going to cause him long-term harm.
What sorts of (other) enchantments are there?
"You do not wish to commune with us?" the fayth wonders.
"—well probably but first I want to ask about Zanarkand."
"It is your final destination, summoner. You will see it when you are ready to."
"No, I mean the other one, the one where there are currently people living and stuff."
"—ah."
"Yu Yevon was a person who has now turned magic being without humanity who attempts to preserve the other Zanarkand which is a dream of the fayth and he uses Sin to do this because apparently there was a war at some point in the past which risked it." Theo rolls his eyes. "Overall: tangled mess of awful."