Is this harder than Dark Souls ?
Yeah, but I think its subjective.
It will feel hard in the beginning since the mechanics are very different from Dark Souls. But as you get used to it it will begin to feel easier.
There is no stamina management, so you can sprint indefinitely and attack continuously. There are only two things you have to keep track of while fighting: Vitality and Posture. Vitality is health, and posture is kind of like poise. Every time you attack an enemy or an enemy attacks you, you take some posture damage, regardless of whether you have deflected, blocked or taken the hit. When the posture bar gets full you or the enemy is "posture broken". In such a case, you can execute a "death blow" on the enemy which will kill them instantly regardless of how much health they have. On case of bosses you may have to do multiple such death blows. Therefore, learning to damage posture is crucial if you want to win fights faster.
Enemies take higher posture damage by deflecting and counter attacking their attacks. Therefore, if you have an enemy who attacks in rapid succession, you will have to deflect quickly and this will fill up the enemy's posture bar quickly, allowing you to end the fight quickly. This is REQUIRED for certain bosses. Timing your deflections correctly will help you finish otherwise hard bosses quickly otherwise its a grind if you try to fight them like Dark Souls.
Thus, combat in Sekiro rewards you if you manage to go face to face with an enemy rather than rolling and backstabbing like Dark Souls.
Also, there are no i-frames when dodging and there is no roll mechanic. Only time you can roll is when you are posture broken and fall to the ground, when that happens you can roll out of the way of the enemy attack and get back to your feet. Lack of i-frames is offset by giving you the ability to jump.
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