Kayen enhanced the spell resistance mechanics in the EQEmu/Server repository, focusing on backend development and combat systems using C++. He implemented a multi-check resist system that allows multiple resistance checks per spell effect, refactored the core resistance calculation logic, and integrated item and alternate advancement bonuses into spell resistances. Additionally, Kayen clarified pet fear resistance behavior and fixed a bug ensuring unresistable spells bypass resistance checks, aligning the server with live game expectations. These changes improved the accuracy and maintainability of the spell system, reduced edge-case inconsistencies, and established a solid foundation for future balance and quality assurance efforts.