It would be easy to quip that Team Ninja's Nioh is the best Souls game since the original Dark Souls. And although that characterization is certainly true, it ignores a bigger, more imporant truth. While From Software's approach to game design is unapologetically woven into Nioh's fabric, Team Ninja's game is far more than a Souls clone. Nioh is in some fundamental ways a more refined and mechanically elaborate product. Although it doesn't achieve mastery in the kinds of subtle environmental storytelling that the Souls games are known for, Nioh's combat and level design take the familiar Souls core concepts in some new, interesting directions.
Based very loosely on the real-life Western samurai William Adams and an unfinished Kurosawa script, Nioh is an action-RPG set in a 17th century Japan that has been overrun by Yokai, creatures from a supernatural realm. Unlike the Souls games, which traffic in Arthurian/Grail mythology and iconography, the various human and supernatural enemies, architecture, weapons and armor are Asian-themed. There are relatively few NPCs of character and consequence in Nioh, whose plot is a competently voiced but relatively bare-bones excuse to engage in combat and exploration.
Like the vast majority of RPGs, killing enemies is rewarded by the collection of some sort of game currency, in this case Amrita (i.e. souls) which is used to level various stats. Instead of merchant NPCs or the ability to fast-travel to vendors, Nioh features a steady stream of randomized loot drops, assuring that there will be a constant supply of consumables. Players lead various little spirits to shrines -- which serve as checkpoints and Souls-like bonfires -- and the spirits can be chosen to weight the loot discovery towards, for example, healing potions instead of Amrita or crafting materials.
At the end of story missions and boss battles, players return to the dojo/hub area, where they may upgrade, buy, and sell weapons and armor, train in various skills, craft new items, or dive into multiplayer as a summoned fighting partner. The hub is also where players select either the next campaign mission, bite-sized sub-mission, or replay an old map or a "dark" variation of it, with new objectives and loot. I did miss Souls fast travel and the ability to return to home-base at will (although there are items to allow it), but Nioh's world is not continuous or connected.
Combat is obviously Nioh's reason for existence and most compelling feature, and it is a far more complex and flexible system than anything in the Souls franchise. In addition to using a dizzying array of weapons and enhancements, players may move freely between a powerful but energy-draining high stance, a fast but weak low stance, and a balanced middle stance as they switch weapons instantly. Spirit animals associated with the player's character gain power over time and eventually become supercharged "living weapons." It takes many hours and some trial and error to master Nioh's combat but it is satisfying and sublime. Somewhat less sublime are Nioh's bosses, however, some of which are outright Souls clones. They're not uniformly bad but a bit disappointing. Likewise, Nioh's level structure -- especially the more labyrinthine dungeons and mines -- is not as clever as in the Souls series and players will encounter a number of traps and hazards that are patently unfair.
Souls games are infamous for their multiplayer invasions, which create both constant tension and the opportunity for an occasionally memorable encounter. In Nioh, players may battle Revenants, which are AI-controlled versions of player characters fallen in battle. Fighting Revenants earns gear and Amrita and another way of honing skills and trying out weapons.
Unlike the Souls games, where items are strategically placed in the environment and generally do not respawn, Nioh is a loot-heavy game and it takes no time at all for the player to be overwhelmed by an inventory of weapons, armor and items. While junk can be "offered" (sold) at shrines, the game's inventory management system is cumbersome and time-consuming to use.
Is Nioh as "difficult" as the Souls games? Yes, and in exactly the same way, which is to note that a lack of stamina management or mental concentration or a greedy desire to get one more hit in will almost invariably result in death. Like the Souls games, the difficulty slider comes from the player gradually leveling to match or exceed the environment and mastering the game's systems. Graphically, Nioh is a dark game and while its environments and levels are often artistic and full of brilliant lighting effects, the color palette is comparatively muted. The musical score is an inventive synthesis of Western and Asian musical textures, motifs and styles but it is understated.
There are dozens of "Souls-like" games but Nioh is by far the most ambitious, polished and successful because it has taken Dark Souls' digital DNA and genuinely evolved it into something both recognizable and different. Nioh owes much to the spirit and mechanics of From's games but there is a lot of Diablo and Ninja Gaiden in there as well. It doesn't matter whether Nioh is derivative or wholly original, it just matters that the game is incredibly well made, challenging, complex and fun.