The Ancient Weapon Poseidon is already known to the vault as “a being who commands the Sea Kings” (Ancient Weapons etymology; Poseidon named Ch. 301). The 0625 beat is the first on-page demonstration of that mechanic in a living person; the 0626 beat is its explicit articulation as a recurring royal legend — both with the name “Poseidon” still withheld.
The payoff
(status pending→confirmed; payoff_chapter 0649. The prior “keep pending — naming is forward (~645+)” note is now satisfied on-page.)
Earliest known caller
(unrecorded)
Why this matters
Oda routinely demonstrates a world-mechanic on-page before he names it — the reader watches Poseidon’s power work a full ~20 chapters before the word is attached to the princess. Routes to Ancient Weapons, Sea Kings, Shirahoshi.