Currently, the highest-level monster that it's possible to fight in Oldschool Runescape—as recorded in the wiki—is Verzik Vitur, with a combat level of 1520.

She has, as of the time of this market's creation, been the highest-level monster in OSRS for almost six years, the longest of any monster in Oldschool Runescape. But this seems unlikely to last forever. So! Market.

This market will resolve Yes if some new fightable monster is added before the end of 2025 which is higher-level than Verzik Vitur; otherwise it will resolve No. Monsters from temporary events, or from nonstandard gameplay modes, or which can't be attacked in conventional combat, won't count for purposes of this market's resolution. A new higher-level form of Verzik Vitur won't count either, if introduced, although if that happens it will push up the level-threshold that any other new monster needs to meet to produce a Yes resolution.