From FortifiedPvP - Official Wiki

Envoys are loot chests that can be found at /spawn and all /warps excluding warp nether, featuring some of the best and most diverse loot in all the land! Envoys will spawn every 25-35 minutes!

Loot Guide

Tier Description
Common Lowest quality loot available
Uncommon More rare items can be obtained
Rare Contains decent loot, a step up from uncommon
Ultimate Much better loot, rare items have a higher chance to be obtained
Legendary Godly loot, the top tier and most valuable of rewards