Nioh 2 has some new mechanics that you will need to get to grips with if you are going to really take it to those demons. One of the new things added to the game is Soul Cores. This can be gotten by defeating powerful Yokai enemies, who will drop them on death.
Soul Cores allow you to do special moves and attacks and can be extremely powerful. They don’t work in the same way as other moves and need a resource called Anima to be able to do them. Anima is earned while you fight, each attack slowly building up your animal bar, which is the third purple bar at the top left of the screen, under your green Stamina bar.
Each Soul Core move will cost a certain amount of Anima, normally four of the smaller bars that make up the overall larger bar. This means you can get two Soul Core moves from a full Anima bar, so you want to use them when they will give you the most benefit.
You can find equipment, armor, and weapons in the game that will have modifiers for how much Anima you can gather, how fast you can get it, and the right items can even increase the size of your Anima bar. You can even add certain attributes to items at the blacksmith if you really want to lean into a build that relies on Soul Core moves. If you are in combat, and desperately need more Anima, you can use Lantern Plant Fruit to replenish it.
You can assign Soul Cores to the two available slots in the Inventory screen, then use R2 and Square, or R2 and Triangle to use them. Anima is also used to perform Burst Counter moves, so you need to be careful about which move you really need in a fight.