A Servant. That was plain. Yet, just what Heroic Spirit was this sinister appearance?

This black knight definitely didn't have any of the "radiance" that bore the other Servants who were already there. Artoria, Diarmuid, and the King of Conquerors Alexander as well as the yet unannounced golden Archer all have that "shine". This is the expression of the pride of a Heroic Spirit. The honor of the legends everyone praises and longs for. This is an essential element of their "noble phantasm".

But the newly appeared black knight had none of it. You could say he was closer to an Assassin. The darkness around the black armor definitely held a "negative surge".

Thus, more than a Heroic Spirit, he might be labelled a vengeful spirit instead...

- Fate/Zero: Act 4, Part 2