It is referring to the Doctor.
The Doctor, at first, assumes the Minotaur is describing itself, but only after the last line does the Doctor realize what we've known all along—that the Minotaur is talking about both itself and the Doctor.
That is, the reason that moment is important is not because of what the viewer sees, but rather the impact it has on the Doctor himself.
It doesn't refer to:
- the others, because it's using the singular
- Amy or Rory, because while both can be argued to be ancient for different reasons, the other grimmer attribute doesn't apply
Furthermore, the series has had a lot of introspection by the Doctor into who he really is: the heroic physician or the "mighty warrior"—this description tallies with the Doctor perfectly, and further serves to demonstrate to him who he really is. It no doubt influences his decision to leave Amy and Rory.