Question #1: In TtD you wrote that Mircea put a ward on Cassie’s power. When did he do that? In the past when she was eleven years old? And why did Billy Joe get the “rush of his life” when he broke through it? Was Cassie’s power as a Pythia so powerful and he wasn’t used to it?

Cassie didn’t have any power to ward at eleven. She was just some seer. But when Agnes started dying, the power started passing, and by the time the senate picked her up, she had more than she knew. But it was mostly Mircea’s power that Billy got a rush from, when he broke through the ward Mircea had put on Cassie when Tomas brought her in, to keep her from accidentally (or on purpose if she figured it out) shifting away from them.

Question #2: And what about later in the books? EtN made it sound like Cassie + the Pythian power might be stronger than Mircea, so does Billy Joe get a “rush” more often, or can’t he access the Pythian power when he feeds from Cassie?

You’re confusing two types of magic.

Magic type #1: Magic occurring in the natural world. This is the kind of magic that, in mass, the ley lines are made from. It is the wild magic of the world that the witches capture to extend their power, or that the Circle gathers more slowly in regular old talismans. It is also manufactured in the bodies of magical humans (which act like talismans).

But here’s the thing: it is manufactured by magical human bodies as a byproduct. Like, for example, a cobra’s body manufactures venom. It is something it makes for a specific purpose that does not involve sustaining life. It might help with that, because venom might keep a cobra alive by defending it against an attacker. But a cobra in a nice, safe zoo could live without it perfectly well. It augments, it doesn’t sustain.

Anyway, that sort of magic is commonly used by magical humans the way we non-magical types use electricity—as a power source for things that make our lives easier. It is used to make wards and shields for defense, to cast spells for offense, to power charms, etc. It is not used to sustain life (see above), or if it is (as with some of the dark mages who use it to greatly augment their life spans) it seriously screws up the user, eventually deforming and/or driving them mad. That’s not what it was designed for.

That’s also, by the way, why Artemis couldn’t just feed from the ley lines to restore her power after she was almost drained fighting the gods. It would have been like a human trying to feed off an electrical circuit. Yes, you can get energy from there, but not the right sort of energy. You need food.

Magic type #2: Life energy. This is what every living thing needs to live, and that animates both magical and non-magical humans. It is also what animates ghosts, who are like the scavengers of the supernatural world, picking up the bits and pieces of life energy living people shed, especially when stressed. It’s why ghosts haunt graveyards—lots of stressed people–and because most ghosts never have the strength to move far from where they end up. Scavengers exist, they usually don’t thrive. The exceptions are those like Billy, who find themselves a special sort of talisman that soaks up life energy, or can get energy draws from someone like Cassie. Those are very rare, and Billy is very, very lucky.

Life energy also animates zombies and feeds demons and vampires. The latter types, because they live so long, may eventually accumulate enough of it to start using it for other things than just living. That’s especially true of master vampires, who get regular contributions of life energy from their extended families. This is why vamp magic tends to manifest differently from human magic. It is more about what they are (faster, stronger, better senses) than what they do (throwing spells, etc).

Even master powers, the special abilities only very high ranking first and second level masters get, usually affect what the vamp is (Mircea’s mental powers, the consul’s transformative abilities, Louis-Cesare’s Veil) rather than what they do, because their power stems, ironically enough, from life energy.

This is also why Mircea, who is technically dead, can heal people, and not just other vamps. He can heal humans, because he can give back life energy to them as well as take it. All vamps can do this to a small degree, but Mircea is especially good at it.

So, to sum up, the pythian power wouldn’t do Billy Joe any good. Just like all the magic in all those magic shops, or in the ley lines for that matter, wouldn’t help him. He needs life energy. But he normally wouldn’t have too much luck robbing a vamp for it, especially not a senior level one, who would easily fight off his attack. That’s why ghosts so rarely attack even regular old humans, because attacking someone uses up a great deal of energy, probably more than you would obtain from them.

But Mircea warded Cassie with a mass of exactly the kind of stuff Billy needs to live, and she wasn’t trying to fight Billy off. Neither was the ward designed to resist an external assault. If was supposed to restrain Cassie’s magic—which it did. Right until it was raided by a guy specifically designed to utilize its abundant power.

So, yeah. Billy got the “rush of his life”!