Way of the Samurai as a series gets high recomendations here.
I can recommend Way of the Samurai 4.
You start off with a sword style you barely know, and work to master it through combat. You can pick up more sword styles to learn along the way.
With time, and effort you can even create your own sword style based off stances, and moves you've learned, and points aquired.
Combat isn't limited to swords. Spears, unarmed combat/claws/knuckledusters, and firearms (they're hardly game breaking) also can be learned.
Swords come in a variety of flavors from jitte, kitchen knives, ninja swords, katana, nodachi, european rapiers, great swords, etc.
Most weapons can be disassembled into component parts, and made into new, potentially stronger weapons. The only weapons that can't be disassembled are jitte, kitchen knives, hand to hand weapons, and firearms, pretty much.
Weapons can be improved upon pretty extensively provided you have the time, materils, and money.
There's dozens and dozens of weapons to be collected in game. Not all are easy to get, or find. If you don't like any of them, as I said, you can build your own from parts of other swords.
Difficulty can be adjusted from easy to one hit kill, but even at low difficulties there's some powerful moves that one should learn to avoid.
When connected to the internet other player's characters can appear in your game as AI, and they might have some excellent weapons.
You can even run into old save versions of your character(s) and duel them.
Swordplay aside, it's full of some Japanese Wackiness, has really nice charcter customization, multiple story paths, side quests, dlc, love interests, the ability to start then defend your own sword school, and it's a pretty charming game all around.
Even if you just wanna screw around, and get into fights it's pretty fun.
Bushido Blade is an old, worthwhile series that seriously needs a reboot.
There's some old 2d fighting games worth mentioning in the Samurai Shodown (not a typo) series, and The Last Blade.