What era of UO is UOX3 based on?

[Xuri]

Various parts of UOX3 such as combat calculations, weapon/armor values etc are based around UO from the time just before (archive.org) the Age of Shadows expansion was released, i.e. circa Publish 15. NPC stats less so, due to the lack of accurate data on such stats from back then.

Note that the combat calculations are hardcoded, but can be overridden through the JavaScript system (or changed in code, of course). The stats of gear and NPCs can be easily tweaked with any text editor, and there are UOX.INI settings that can globally increase/decrease combat difficulty and/or speed.