Reign could feel the mana surrounding them easily thanks to the mana organ, even with his eyes closed he could still feel the mana moving and he could spot movements, not to mention that everybody has mana in their body so trying to sneak up to Reign was pretty much useless right now. Of course, people like Shadow were able to as their abilities worked a bit differently so it was easier for him when he blends in with a shadow his mana also disappears alongside him.
The best thing about the mana organ was the fact that everybody had it, but the difference with Reign was the fact that he already started at the peak of its phase. Usually, a mana organ will start off at the beginning phase like Reign's, but the difference is that people will usually have to slowly improve and get more mana with which the mana organ will grow as well.
