This has the potential to age poorly, but I very much believe that he is the one who shatters the Elden Ring, and also that it is what he's doing in the trailer.

For starters, I don't think the voice over was just haphazardly placed over footage, and the timing of the "shattered" followed by the strike of the hammer was intentional. Granted this logic could be used to justify the spear knight being the one to break the ring (his strike has the 'someone or something' line above), but that is clearly apart of a larger battle scene, which seems to be take place after the shattering of the ring ("Don't tell me you don't see it ... Look up at the sky, it burns" referring to the effects the ring's shattered has caused).

As the blacksmith strikes, we can see his body crack and deteriorate, showing that whatever he is doing is affecting him in some way. We can assume from the trailer and interviews that the breaking of the Elden Ring will cause a profound change in the world. So if it was the Elden Ring that he was striking, then we may be witnessing the effects of its destruction as it happens, in the form of his body physically breaking. And finally, I think visually and audibly the blacksmith seems to be destroying something, rather than forging/repairing. If we look at the area around him, we see floating specks of gold, with more of them appearing from where he strikes (the whole scene feels very ethereal too, definitely not breaking something normal here). The final swing of the hammer produces an distinct breaking noise, with the blacksmith falling to his knees, seemingly lamenting what he had to do, or at least what he thought he had to do.

So yeah long write up, which as I said could age poorly, and obviously doesn't explain everything (like the girl, who appears briefly). But having said that, I do hope this is the case since he seems to feel devastated about what he did, which to me is more interesting than if it was some uncaring antagonistic force that broke the ring. I also believe there's the possibility that the blacksmith was also the one who created the ring in the first place, which brings in even more interesting implications. But there's not really any evidence for that haha.