Effective Regalia is a mod that aims to add ways to improve the PvP/PvE experience for both single-player and multiplayer scenarios. This mod adds, as it implies, several Regalia variants. For those who don't know what 'regalia' means, it is basically a fancy word for bracelet or bangle.
Depending on the variant of Regalia that you have in your offhand, you will gain different buffs, and potentially debuffs if your regalia is at low durability.
You cannot find regalia naturally in loot tables, you only craft the regalia; the recipes vary but all follow similar constructs. You will need to either find or craft a relic associated with the regalia you are attempting to craft. Once you have obtained the relic, you will be able to craft that regalia variant!