Forsaken Cainhurst Castle is a completely optional area, but if you manage to find this frozen kingdom, be prepared for a challenge. Atop the castle roof awaits Martyr Logarius.
Logarius has a wide array of attacks at his disposal. The most basic are slashes and slams of his giant scythe. These are best avoided by dodging around him. He may also summon an ethereal skull directly in front of him that will then rush you. Another sidestep will do the trick, but the window here is brief. He may also charge up for a massive area-of-effect blast - keep your distance on this one - that also increases his attack and speed for the rest of the fight.
When it comes to range, Logarius has a few options. The first is to summon more of those skulls that home in on you, and the best way to avoid damage is to stay up against him. He may also leap into the air for a pouncing attack. Roll away to avoid damage, then use the window to get in some hits of your own. Finally, he may jam his sword into the ground and summon a cloud of flying blades. These will home in on you too, and the cloud will not go away until you attack the sword planted in the ground. Keep all these attacks in mind, and you'll have a decent shot at taking down the Martyr Logarius.