XY-era cards (including Brilliant Stars) remain in the early phase of their market cycle the original collector cohort is in their teens / early-20s
Influencer & Market Hype Popular YouTubers, streamers, and sellers have highlighted Mega cards, fueling short-term spikes in demand
Its passive effect is decent, although, it isn't often you'll be banishing your Maliss Trap cards as you generally want to banish your monsters instead
That said, theres no guarantee that this price spike will last, even until the release of Final Fantasy
Do you build around them or play them incidentally
Cards from 'The First Chapter', 'Rise of the Floodborne', 'Into the Inklands', and 'Ursula's Return' are not legal in the game's Core Constructed format, except when the card has been reprinted in a later set