From what I've been able to ascertain, Monstrum was a 2018 Korean release that never found U.S. distribution until now: it'll be offering as a motion picture release to the Shudder outlet (not familiar with it, but, then again, the wifey and I don't stream much). It's also being billed as a Shudder Original, which may or may not be a good thing: putting one's name on something generally indicates that the service believes in the quality, and this one's preview looks very solid. The release is posted at FirstShowing.Net, but the details are fairly slim.
Check out this preview, though. Pretty slick.