Headless is an optional mini-boss located in a number of locations in Sekiro: Shadows Die Twice.
This particular Headless encounter takes place in the grounds of the Ashina Castle, but it can only be accessed once you have defeated the Corrupted Monk and unlocked the Mibu Breathing Technique, which allows you to dive underwater. Once you have that, head to the Upper Tower – Antechamber Sculptor’s Idol, head up the stairs and kill the first two enemies. Then, jump over the rail to the room below where you’ve hopefully already killed the Chained Ogre. Now, head out of the back exit (rather than the main entrance into the castle) and you’ll come to two enemies who are looking out over the lake with their backs to you. Kill these two and then it’s time for the boss fight.
Jump into the water and kill the two fish on the surface. Then, dive underwater and you’ll see the Headless stood right there waiting for you. Unlike other Headless fights, you can’t use things like Divine Confetti or your Phoenix’s Lilac Umbrella while you’re underwater, but you can use your Mottled Purple Gourd. While that may initially sound like this fight will be harder, it really isn’t. Thankfully, the fact that we’re underwater means that the Headless can’t slow us down, so we have a big speed advantage. Because of that, our tactic is simple – target him and swim around him in a circle by pressing the sprint button, swim towards him once you’re behind him, get two or three hits in, then swim back out and start circling again. Rinse and repeat.
It honestly is as straight forward as that. Watch out for his Perilous/Red kanji attack which sees him pull water from around him to draw you in, but to be honest you’ll avoid this easily by just following the tactic of swimming around him in a circle.
Make sure you have your Mottled Purple Gourd equipped just in case you do get caught by any attacks, as you’ll take considerable Terror damage. If that happens, back out and swim around him until there’s a window of opportunity to use the Gourd, then use it and get moving again straight away so that you avoid the next attack.
Keep circling and attacking when you get the chance and you’ll soon open Headless up for a Deathblow. Thankfully these underwater fights only take one Deathblow to kill, so hitting this one will end the fight.
Killing this particular Headless will earn you Ungo’s Spiritfall, which can be used to reduce Vitality damage taken from physical attacks.