Something about killing off villains one by one that normally makes it easier for the heroes. Obviously, therefore, the villains have to start out dismissive and contemptuous, and get angrier and more willing to expend resources to get at our heroine and her friends, so as to amp up the danger. (It's not like killing their allies will make them happier. Even the most vile villain will object to losing support.) At the very end, before the climax, the lone survivor has to go all out on them.
So I threw in a spell she can cast that will hamper them enormously. Except -- you know, they were going to run into two armies about to be on a battlefield -- and the spell would prevent that. I can't move it after because it would be anticlimax, and removing it would require massively reworking a thread. (Pulling the thread out entirely would require an entirely new one to deal with a secondary hero's happy ending.)
Hmmm. . . hmmm. . . you know if I gave this tertiary character a magical ability that does exist in this universe -- and I used it to interact with the spell, which had some effects the villainess did not realize at the time -- it could even make the plot smoother --
Which meant that today's writing consisted of going back and putting in things that were needed for support.