A Song of Ice and Black Blood

Slade is held captive by his family, the royals, seen as an abomination. Bleeding black ice and having a terrible karma surrounding him, causing chaos and destruction to all those around him, he is acknowledged as nothing more than a threat. He also has a masochistic, malicious nature, uncontrollable. He's only brought out by his captors to be used as an assassin, killing those against his family.

When someone comes against the castle and breaks him free, what intentions do they harbor? What sort of danger is the city in? Worse yet, what history do these stony, cracked walls just barely contain?