Glamour as a magic path is new to Dominions 6, but the Glamour trait has been in the series for ages. This fay and jann art to shape appearances is abstracted to two different functions: one in combat, and one outside combat.
Glamour in combat does the same thing as the Mirror Image spell. It projects an equal image of the bearer right next to them. For those with not enough Glamour magic skill to have 1 (and/or for troops), they project one image. For those who do have levels, they project 2 per level, plus 2.
Mirror Images essentially divide the chance to be hit across every image. For example, troops with Glamour have only a 50% chance to be hit. 1 mages would have only a one-in-three chance to be hit, 2 mages would have only a one-in-five chance to be hit, and so on.
In Dominions 5, Glamour abruptly ended in battle once the user took even a single point of damage, as long as it wasn't incidental damage from Blood Bond or Blood Vengeance. It also only protected against melee hits. Thus, short-ranged missile combat and area-of-effect magic were typical counters against the glamourous.
In Dominions 6, each individual image persists until it takes a single point of damage. It is not yet clear if images now protect against missile hits, though it definitely doesn't protect against mind-attacking spells. In all likelihood, area-of-effect magic will hit all images (and the image-maker). Regardless, rather than working in an "all-or-nothing" manner with the possibility of infinite survival, Glamour now has a maximum amount of successful hits to "negate" each battle (but doesn't disappear all at once).
Glamour is more distinct outside of combat, where it allows fellows to hide their weapons and attire. In their own lands, they are invisible on the strategic map and will not be mentioned in scouting reports, as long as the fellow searching for them doesn't have Spirit Sight. Glamourous fellows actively hiding in their own provinces are especially imperceivable.
The Glamour trait makes the infamous "elfing" strategy all the more dangerous, as it hides most of the preparation and buildup from most nations. Once within enemy lands, however, a glamourous unit is no more secretive than anyone else with the same Stealthy score.