Agile Guardians spawn from the caskets in the Graverobber's Cavern, fighting alongside Robust Guardians. Befitting of their name, they are quite fast with their extra movement speed and will focus on reaching other caskets to wake up more undead. This is especially hazardous if the Ornate Mirror flaming trap is triggered.
In Tactician difficulty and above, Agile Guardians erroneously have the Archery passive applied twice, totalling to a bonus of +4 to ranged weapon attacks.