Dawn of Dreams & possibly the first game: Fortinbras, God of Light, was born out of the primordial chaos as the first God, and sired the Genma race, making them to be malevolently evil beings without an iota of compassion. Fortinbras then created the human race but designated them to be food for the monstrous Genma, with their souls devoured to further empower the beasts. Also a cruel manipulator, Fortinbras made deals with humans power-hungry enough to sell their souls to him and handed out contracts to countless conquerors throughout history, whom Fortinbras viewed as his own disposable pawns. A megalomaniac mad with power, Fortinbras enables countless atrocities, as seen in his empowering of Oda Nobunaga and the nature of one of his creations, Guildenstern, hellbent on keeping all of humanity as nothing but slaves at best or food and victims for the Genma at worst.
First three games:
In life, Oda Nobunaga was a brutal warlord. After his death and resurrection, he becomes far worse. Planning to achieve immortality by drinking an innocent princess's blood, Nobunaga, who serves as the Big Bad, soon becomes the king of all demons to replace the then-dead Demon King Fortinbras. Leading his forces, Nobunaga embarks on a vicious campaign through Japan, slaughtering civilians. Nobunaga also wipes out the Yagyu village, leaving only one survivor. Stopping at nothing to dominate all Japan, Nobunaga is a stark reminder that humans can be just as evil as any demon.
Guildenstern—only mentioned in the second game—the Mad Scientist of the Genma and Nobunaga's right-hand demon, is a monster who enjoys the taste of organs and crafting new monsters to extend the power of the Genma. Guildenstern is the one to transform and resurrect Nobunaga and to facilitate the destruction his forces cause. In the third game, Guildenstern opens a rift to modern-day Paris and leads the Genma forces to slaughter everything they find. When he is found by Samanosuke in Paris, Guildenstern is even shown about to devour the organs of a child, stating he just loves how delicious and soft children are.
Demonic Spiders: Many Genma, except for the basic zombie grunt, are this, especially in Dawn of Dreams in the Demonic Realms, where you'll find lots of nasty Ninja, Giant Mooks and Invisible Monsters and no sense of Mook Chivalry. Airborne Mooks are particularly bad since most characters lack the appropriate means to deal with them.
Epileptic Trees: Many theories have risen among fans as to why only the first game received a remaster as opposed to all four, especially since Capcom's twoother big Hack and Slash series both received full HD collections of their older titles. Some speculate that since the playable characters are modeled after real actors, Capcom may have to pay royalty fees to reuse their likenesses and did not want to spend more than they had to, thus only settling on the original game. Others believe that since the franchise laid dormant for so long, they didn't want to risk using their resources on something consumers may not want and decided to test the waters first. On a more cynical note, some fans also think that it's just Capcom being lazy, or that they're only cashing in on the resurgence of samurai action games that began in the late 2010s.
Fake Difficulty: The series' origins as "Resident Evil in fuedal Japan" plays into its "challenge" due to the use of fixed camera angles and tank controls, at least in the first two titles. Because it is far more action-oriented than Resident Evil, those fixed angles can cause more problems than a full 3D game would have, with many moments of enemies (often ranged enemies) attacking from off screen. You might be trying to time that perfect Issen, but you can't see that your foe is already attacking from outside your vision, and you get hit. Demon Siege alleviates this for the most part by using full 3D environments and a "dynamic fixed" camera, a la Dino Crisis, as well as allowing camera-relative analog stick movement as an option over tank controls.
Ultimate modes in the original games usually required you beat the hardest difficulty to unlock them and you'll be laughing your way all the way to the ending upon starting them; the first game's ultimate mode for example had you start the game with infinite arrows and fire arrows, infinite bullets and burst bullets, 99 Soul Absorbers, infinite magic power and the Bishamon Sword.
Samurai's Destiny and Demon's Siege could give the player a chance to obtain either a white necklace/vest in the former/latter which allowed the player to regain health as long as they stood still. Yeah, it's a pace-breaker if you got hurt a lot, but if you don't mind waiting for a full recovery, these accessories were amazing for the player.
The Black Necklace/Vest. They're ultra difficult to obtain, but the ability to automatically use an Issen with every attack, at a small cost of health, is unbelievably powerful. And, since enemies always drop healing souls when killed by an Issen, the health cost is effectively neutralized.
Goddamn Bats: Archers and other ranged enemies. They will always spawn out of reach, their attacks don't do much damage compared to most melee mooks, they can die with to single arrow, and they will interrupt you with every hit. Alone, they're pathetic and a nuisance since it means going into your inventory to equip the bow to kill them. In mobs, they're even more annoying by staggering you while you're set upon by much stronger and deadly enemies. Even mobs solely comprised of archers are more irritating than challenging since they love to pelt you with staggering arrows in rapid succession.
Moral Event Horizon: Even though he was already a literal demon by the time it happened, it bears mentioning that Nobunaga actually did burn down the temples at Mt. Hiei in Real Life, so he qualifies for this both in the games and in the eyes of many historians. In fact, this act is one of the main reasons he receives a Historical Villain Upgrade anywhere he appears.
The fixed camera sometimes makes it hard to notice the enemies.
The most common complaint the first two games got is the use of the dpad rather than the left analogue to move. This is now fixed in the remaster of the first game, allowing players to use the analog stick to move.
Sequel Difficulty Spike: Issen appears to have been nerfed in 3 to make it harder to spam. This in itself is not a big deal, except that acquiring the Ako's Black Vest means completing the Critical tutorial: enemies can only be killed via critical hits. Deflect criticals are much easier, since your character is at least protected from the blow. However, this tutorial requires that you leave yourself wide open to attack. Not for nothing is this considered the trickiest level of any Onimusha game.