Tenchu Souls

I've seen Sekiro: Shadows Die Twice compared quite a bit to Dark Souls, Demon's Souls and Bloodborne.  What I haven't seen much of though, is comparisons to the Tenchu series.  Maybe because it's an old IP that hasn't had a sequel in over a decade...or maybe it's because the best entries in the franchise were back on the PS1 (and as such have bad controls - and even worse graphics - by modern standards).  Either way, the fundamental gameplay in the Tenchu series is a very close match to Sekiro...not much of a surprise when you consider that all of these games were created by one developer, FromSoftware.

Basically, both Sekiro and Tenchu have stealth mechanics which the player can utilize to pick off enemies one-by-one.  If that fails, they can always fall back on face-to-face combat or retreat and try stealth again.  Punctuating this loop are occasional (mini-)boss encounters wherein the player must fight without the benefit of stealth.  Even little details like the grappling hook, sub-weapons, and support item shops can be found in both games.  A few of locations and objectives are also very similar:

• Buddhist monks turned into undead cultists...check! 
• Important "flower" item on top of a mountain...check!
• Difficult to navigate forest with sniper archers...check!

Some of the enemy types as well:

• Hard to stealth by dogs...check!
• Big, fat dudes with clubs...check!
• Naginata wielding women...check! 

The last boss in the original Tenchu, much like the last boss in Sekiro, is ridiculously hard as well.

Of course a lot of the above are just staples of the genre and setting.  Sekiro might copy a bit, but it also improves on the concepts of Tenchu in pretty much every way.  The environments are far more detailed, the animation is far more fluid, and the world is interconnected rather than a sequence of compartmentalized missions.  In terms of audio, music in Tenchu is surprisingly good.  Sekiro definitely outdoes it with regards to sound effects though.  Voice acting is hard to compare...Sekiro allows for the original language which is almost always the ideal way to play a game.  That's not to say the English dubovers are bad...it's just that inevitably some things get lost in translation.  Meanwhile, the original Tenchu is a shining example of so-bad-it's-good voicework.  The sequels attempted to improve the quality of the English voice acting, but the results were pretty mixed.

So, if I were to compare Sekiro to any other game, my first pick would be Tenchu.  After that, I might go with Legend of Zelda in that both it and Sekiro use non-numerical RPG mechanics and a semi-open world design.  Only after those would I then try to draw comparisons to Bloodborne.  Dark Souls and Demon's Souls are, at most, fourth on my list of related games.