Dairos said that you needed to do battle in Theobomos to earn a Sanctum Decoration. Following his instructions, you defeated Cave Bloodwings, Forktail Sylphens, Thornback Hornskulls, Sand Tongue Hornskulls, and Hillock Abexes. After confirming the evidence, Dairos gave you one Sanctum Decoration and told you to complete more heroic deeds.