Hwarangs are taken straight from the history books, like most of Atlantica's mercenaries, as they were an elite military unit from 10th century Silla (an ancient Korean kingdom.) In the game they will be represented as potent archers with a high offensive capability but low defense. Hwarang units also have the option of sacrificing themselves in battle to buff the rest of the mercenaries under your command.
Hwarangs aren't the only thing coming with this patch, however, as the Atlantica team has made the difficult decision to increase the minimum system requirements in order to cut down on load times and increase client optimization. This move has been made in order to make sure the client can perform in areas with high population counts, as that has been a hang up of the game in the past. Only a small minority of users should be affected by this change, but it will be a wealth of benefits to all other users operating in high population areas.
The full patch notes are available on Atlantica's website, if you wish to delve into the many changes and fixes.