Okay! I would say strap in for some boring technical the-nature-of-fantasy-writing stuff.
- How did Belial make Tessa with his curse?
We already know Belial can, in fact, walk on the earth if he possesses someone. In the case of possessing an ordinary human, the body falls apart quickly. If he possesses a demon it lasts longer, and he could wander about for a while as a demon if he liked, he’s just not interested. He’s obsessed with the idea of being on the earth in a human body: i.e. James. Anyway, he possessed the Eidolon demon that took the form of Richard Gray, Tessa’s father.
2) Was Belial’s debt to Jem originally planned as something other than passing on a message?
It took me a while to try to understand this question, and I’m still not totally sure I do. I thought it was a question about what the favor was, but I think it’s “Did you ever plan for Belial to owe Jem for some other reason?" To which the answer is no. It was always planned to hinge on the deal that is made re: the Herondales. While Jem frames it as "passing on a message” that is in fact, not what the favor was.
3) Why would Belial give powers to James and esp Lucie when they could use them against him (and in the latter’s case, he didn’t care about her)? I just still can’t understand why they would be taken away with his death or why he’d intentionally bother with that.
These seem like two different questions, so for the first one: Belial didn’t give powers to James and Lucie in the way you seem to be framing it. He gave a specific power to Grace, which is why Grace, you know, remembers that happening. James and Lucie just have powers because they are related to Belial. Same reason Tessa has her power. Belial had no idea what the specifics of all his mortal descendants would be—how could he? Shadowhunter/demon hybrids had never existed before. He may have thought they might have powers of some sort, but he did not handcraft them and send them to James and Lucie for Demon Christmas. :-)
As for why, I guess, Belial didn’t kill them in the womb or something (assuming he could have done that) — this seems to be the question, “Why would Belial ever make a mistake or take a risk?” And the answer is, “Why wouldn’t that happen?” Being a demon doesn’t insulate Belial from having flaws or being in error (some might say that’s the essential nature of demons.) It’s a bit like asking why Satan made war on God in Paradise Lost when he was certain to lose because you know, God. Milton kind of wrote his entire epic poem to answer that question. And (Cliff Notes): the answer was pride. Pride blinded Lucifer, and if Lucifer can be blinded by pride, then Belial can also be blinded by his own flaws, specifically hubris and a misunderstanding of the nature of human beings. While Belial knew James and Lucie had powers, he never thought those powers posed a significant risk to him and he wrongly assumed he would always be more powerful than they were and easily able to turn their powers to his advantage. Jem explains Belial’s error fairly plainly:
“There was also the great weakness of demons: they did not understand either love or faith. Belial had underestimated not just Cordelia, Lucie, and James, but also their friends, and what they would all do for each other.”
As for why J&L’s powers vanished with his death: I think there is a confusion here between two kinds of magic. Rules-based magic, and numinous magic. Rules-based magic being what it sounds like, and numinous magic being magic that has an almost mystical element that cannot be explained like a math problem but is rooted in the resonance of human experience, of myth and fairy tale. Rules-based magic needs to be explained to us in fiction. Numinous magic we understand because we’re human and we know. (Jo Walton has a good essay on the two kinds of magic and the ways they work together and apart here.)
Neither is a better kind of magic in fiction than the other and most good fantasy contains both. An example: The Ring in Lord of the Rings. It has a numinous power: it exerts an evil influence, but that evil influence doesn’t work in a specific, ordered way, and it works differently in different situations and on different people, often without an obvious reason why. But it makes sense, as long as you keep the essential nature of the Ring in mind. The Ring also has another power. If you put it on, you become invisible. The end. Works the same for everyone. The Ring has both numinous and rules-based powers.
If you are looking for a math-type answer to why James and Lucie’s powers vanished with Belial’s death, there is not one, as their powers were inherently numinous to begin with, and we already have both rules-based and numinous magic in the Shadowhunter world. In fact, we already have examples of magic coming apart when the magic-creator dies in the Shadowhunter world. When Sebastian dies, the Endarkened die with him. When Johnny Rook dies, his wards fail.
To go back to Lord of the Rings, when Sauron dies, his fortress of Barad-dûr collapses. This is not because it was architecturally unsound. It is because we understand things like Sauron’s fortress to be the expression of someone’s will in the world, and when they die, their will is broken.
The breaking or undoing of that will is concretized in fiction (just like all metaphor is) — so while James and Lucie couldn’t have predicted that their powers would vanish with Belial’s death as their situation (being a Great Demon’s Shadowhunter grandchildren) is literally unprecedented, it makes sense to them that their powers would go when he went. They don’t seek a rules-based explanation since 1) they already live in a world where magic sometimes disappears or collapses when the magic-creator dies and 2) they don’t need one. As James says, “Not because [the gun] no longer fires at my command, though that is true—but because it only ever worked for me because of Belial, and Belial is gone.”
