Sekiro has been doing exceptionally well for Activision and FromSoftware. Kadokawa, parent company of FromSoftware, has confirmed that the samurai action title sold around 3.8 million copies as of June 30, 2019. That puts it in the range of being one of the most successful new IPs of the current generation.
It's hard not to see why Sekiro has sold that well, either. Whether or not you believe the game is "too difficult," From's previous games have built up a pretty massive cult since Demon's Souls released in 2009. With the prevailing idea that From games are "brutal as hell," people looking for more challenging titles flock to each release to get their masochistic fix. It also doesn't hurt that samurai titles seem to be coming back in vogue, what with Nioh 2 and Ghost of Tsushima still in the pipeline.
This is a well-deserved victory for FromSoftware. I'm happy to see the company go from creating niche titles to well respected old-school hits. Whatever the future brings, I hope their pedigree for quality continues.