Plan: Talk to Byakuren (or whatever other magician) about whether dreams like this are remotely reliable sources of information for such things. The 'Celestial Hierarchy' and Forest of Magic scene are both worrying and informative. It's a good question whether repairing the sword would be a good idea at this point - such as whether/how well it could be used against its former master, directly or indirectly.
Also possible: Bringing an appropriate army to question/challenge Yukari, and base what to do with her depending on her response to/helpfulness with a mention of that Celestial Hierarchy, if she has any reason to be aware of what that is (at least, beyond what can be readily discovered from Patchouli's library, or maybe Eintei if they're willing to actually be helpful, though that might be a lower priority than Yukari what with Tewi being a derp and all).
>"Well, thank you for being helpful, at least."
>Any idea who would be up in the Temple at this hour?