Owen Watson has a nice in-depth look at Kendall Graveman's repertoire at FanGraphs. That sinker looks nasty, and if there's one pitch, besides Mariano Rivera's cutter, on which man can live alone, it's a nasty sinker. (N.B.: That's an untested statement. It's probably not true. But it might be!)

One note I'd add is that Watson writes that Graveman's sinker "has a lot of sink, but it also has a ton of arm-side run, so much so that it looks at times like a two-seam fastball" and I think that's a distinction without a difference. A sinker and a two-seam fastball are, I believe, the same pitch. Everyone's grip and release differs a little, surely, and everyone's ball behaves differently, on a three-dimensional spectrum of velocity, arm-side run, and sink, but my understanding is that broadly speaking they're the same pitch. Some we might prefer to term a "two-seam fastball" because they're heavier on run than sink, so they seem more "fastball-y," as Watson alludes to with Graveman's pitch, and some we might prefer the opposite. But in the end, the man working in the public sphere who knows more about categorizing pitches than anybody else I know, Harry Pavlidis, eschews any distinction—Pitch Info, which powers the categorizations at Brooks Baseball, has only the "sinker," not the "two-seam fastball."

None of this is to call Watson out, I want to note; I highly recommend the article. I just think it's worth all being on the same page about terminology.