It is an achievement in the first story instance: Behind Enemy Lines.
You need to be spotted by dominion soldiers and convince them to let you go 10 times.
Does anyone understand how it works? Seems random to me (and so it's really annoying to retry it).
Some suggest (in guide) it depends on armor types (heavy, leather), but it doesn't work for me.
Carcharoth Lucian/Mini Chibii
Ruin of Surmia world
PvE : [CdL] Les Chasseurs De Légendes
WvW : [MIMs] Mobile Ingénieuse Et Marteau