Its in-game description reads "Legendary armlet made of sunlight changed into metal." It is a very powerful armband, especially against Undead monsters due to his high stats. And when it is fully charged, it is even more powerful. You can only find the Sun Armlet in dungeons' chests, it cannot be invented or bought in any way. This brassard is a high-tier weapon (Tier 7) : it'll gain 6 Synthesis Points (SP) per level-up.