A shadow knight rose up,
holding an enchanted sword named after me.
That blade will challenge the devil's reign.
The enchanted sword must be the Sparda. Either that, or it's the Alastor, since Alastor is also the name of a demon in existing religion. So somebody who is NOT Sparda rose up holding his sword. That might be Vergil - he did use it in DMC3 for just a few minutes... I think.
The weird thing is that it says ''that blade will challenge the devil's reign''. So either it hasn't happened yet, or this part of the poem was written before DMC4 - in which the Sparda sword refused to give Sanctus power.
Destroying the minions of darkness
that stand in his path,
the knight finally faced the strongest demon.
But his strength was no match
for the devil's dark wrath.
He was defeated and fell into oblivion.
The first three lines sound like Sparda. The rest sounds like Vergil, since Sparda wasn't defeated by Mundus (I think, though I have my doubts - after all, Sparda was sealed away and returned in the 20th century to marry Eva).
The people offered up their prayers,
believing in the day when the darkness would pass,
and sang songs of remembrance to the fallen knight.
The people are probably the Order's subjects, or all the people in Fortuna. I'm not sure this part even has anything to do with the last - why would people pray for Vergil, who they hardly know, and has never tried to 'end the darkness'? This is probably about Sparda somehow.
These prayers became our strength, our miracle,
and by this miracle the shadow knight was reborn,
and once again fought the devil's power.
This could be about Nero. In that sense, he's Sparda reborn. But it could also mean the Savior (the giant statue), whom the Order thought was good, and was literally meant to look like Sparda.
As the shadow knight said,
the darkness has cleared.
If this pertains to the last part, then it implies the Savior has saved the world by defeating the demons/sealing the hell gates. This could mean that Nero and Dante never got praised for what they did. That people still thought they were the bad guys.
If not, then Nero got the praise for clearing the darkness.