+C: Sword and Cornett [Manga]

From Shinmakoku Scanlations:

Once, there was a hero who vanquished the darkness on this land and liberated the people from it. Then he built a kingdom, and had his descendants to rule as kings. Although they had ruled the land for so many years, the truth is the politics is being centered on the elders from each region of the kingdom.
This seemingly peaceful kingdom was then disturbed by the mysterious death of the next successor to the throne; the death started to involve all of those who have the rights to succeed the throne to become the next king.

Belca Noctircus is the son of a commoner and the King Noctircus. He is the third prince in the royal Noctircus family and is third in line for the throne. Soon after coming back from abroad, his brother, the crown prince Hector Noctircus, revealed his amibition to create peace with the Amonel (a race considered to be monsters); but mysteriously, he dies from a chronic disease. Belca and his second older brother, Orcelito, discovered the conspiracy behind Hector’s death, and Belca was nearly killed because of that. However, he was saved by a wandering bard/scholar named Eco, who agreed to help Belca out, but is Eco really who he seems to be? Meanwhile Orcelito is captured by Lagen, whose son, a young nobleman, locks him up and manipulates Orcelito to do things by Lagen’s bidding and wishes…