You get this stigma from a quest. If you're an asmodian the quest is called [Group] Senegal's Arrow, you pick it up around the Subterranea Observation Post, and complete it killing the elites outside the Udas entrance for the quest item.
If you're elyos then I don't know what the elyos version of the quest is called, but it should be in the equivalent area, i hope this helps.