Velis has been three different people to three different search parties, and every one of them was standing exactly where she wanted them to be looking. The cavern's flooded mirrors do the rest of the lying for her.
She blinks a short distance and gains real Concealment for several seconds - a genuine miss-chance effect, not just a screen flash. A Reflection charge banks (cap 3). No prior mechanic here is fed by the boss's OWN movement rather than the party's actions against her.
Her epic spell detonates at her new position, the cavern's surface flashing to scalding vapor around her - reflavored from a literal fireball to fit the flooded setting - and the charge resets to 0.
Flooded Cavern's real ceiling stood at CR15 (Kessa Wavemother); Velis is pitched one notch above at CR16, into the area's last remaining Sparky slot (enc09, 9/10 before this boss).
She is the bestiary's first Beguiler - a real dedicated arcane caster (classes.2da SpellCaster=1) built entirely around illusion and enchantment rather than raw force, cloned from the module's own bele_seahag donor for its already amphibious, off-human appearance fitting a cavern built on captured seawater.
Real Epic Spell: Hellball (id 636/876) - reflavored the Boiling Reflection so the cavern's own flooded surface does the detonating rather than a literal fireball out of nowhere.