The Obsidian Tower

To save his sister, a desperate knight murders his king and frees the demons from hell.  While the demons ravage the country, the orphaned son of the high priest must team up with the dead king's cousin to defeat them. But the two heroes' loathing for each other is only outstripped by their desire for power. While the priest falls to the demons' embrace, the king's cousin struggles with the knowledge that the kingdom he risks his life to save will be ruled by someone else.