How to get the DLC weapons in Fire Emblem Fortune's Weave

Fire Emblem: Fortune's Weave 's DLC weapons give your army a sizeable advantage, though they do come with a few drawbacks. Obtaining the weapons involves completing a new subquest, and each of the four playable Flame Lords gets a special weapon. There's a bit of timeline funkiness involved in getting those weapons to Part 2 or beyond if you've already completed someone's Part 1. It's worth the hassle, though.