I Cast a Spell on You

Question: If they have the XP points, can a character with Magic make a spell up on the fly and then pay for it at the end of the session?

Hmm, this is one where we may not be a lot of help to you, so we apologize in advance. While the Scion books encourage players to invent new spells for the Magic purview, we actually don't allow that; Magic-users are limited to the spells on our website (although we do our best to listen to player ideas for new powers and add them to the game if they're good ones).

We do that for a few reasons, but the major one is simply that writing spells is really, really hard. Seriously. A given spell has to be a power level commensurate with all other boons at that level, has to make mythic and story sense as something that someone in the Magic purview (which revolves around manipulating Fate in various ways) would have, and needs to avoid duplicating other powers in the game, of which there are oodles. It takes us a good long while to write new boons that we think are balanced, useful and appropriate for the game, and while our players are smart and creative people, most of the time we don't really want to deal with them continually presenting us with new powers that we have to try to vet, edit and/or reject. The intent of the RAW is noble - to provide the purview room to grow, since theoretically the ways one could monkey about with Fate are endless - but putting it on the shoulders of a player, who is naturally biased toward his character, seldom yields good results.

We also tend to feel that if a given spell is awesome and resonant enough to be allowed for one PC, it should be available to all who use the Magic purview, so we'd rather just work on writing new boons to add to the pile that they have to choose from instead of working the proces sin reverse. There's also the issue of Magic being the only purview that allows a player to write her own new powers, and that's pretty shitty for all those non-Magic users in the band who are sitting around wondering why they don't get to invent new superpowers whenever they don't have the right option on their toolbelt.

So, the short answer: no, because in our games you can never make up a spell on your own so the question is moot.

When it comes to whether or not you can buy powers with XP in the middle of the game, however, that we do have guidelines for. We allow Scions to purchase any power they wish in the middle of play as long as they are Legend 5 or higher; Heroes don't yet have the more-divine-than-human flexibility that Demigods do, so they have to develop more slowly, but everyone of higher Legend can grab Halo of Fire in the middle of a combat and bust off surprise moves to their heart's content. However, we would never allow a Scion to pay XP at the end of the game; if you can't afford it right now when you want to buy it, you can't get it, period. If you would have to wait until the end of the game to have enough XP to purchase it, you'll also have to wait until then to officially buy it, and it will be the next game before you can use it in play.

This is a house rule, though - Storytellers always run how and when characters can spend XP differently, so that'll be up to the sole discretion of whomever's helming your table. Some allow you to spend XP whenever you have it, however you want, while others require it to be spent only at the end of a game, only at the beginning of a game, or even only at pre-designated points in the story or when major events happen. Check in with your Storyteller if you're not sure of the rules there, and if you are the Storyteller, roll with whatever you think will make the most sense for your game.