Yhorm the Giant is located at the bottom of the Profaned Capital. This fight isn’t overly compicated, but is harder to pull off than one would think. It may take you a couple of tries to get it down. After entering the boss fog you will want to head to the item laying near the throne at the back of the room. It is the Storm Ruler greatsword and is all but required for this fight. You can kill him without it, but it would take you a LONG LONG time. It is rather difficult to pick up the item and equip it without getting killed, so I recommend using a ring of sacrifice on your first attempt if you have any amount of Souls on you worth saving. Having equipped the sword you will need to hold L2 for 5 or so seconds to charge up its attack. Pressing R2 while still holding L2 will unleash a blast that severely damages Yhorm. You can also charge the weapon for 5 seconds and then move normally, attack regularly and when you are ready to unleash another blast simply press and hold L2 and hit R1 to fire it off.
You will need to learn how to do this as you will not always have time to charge the attack and fire it off without getting hit. The challenge in the fight is dodging Yhorms attacks in between charges, creating enough distance between you and getting enough time to fire off the weapon. Since 1 hit from the boss will most likely kill you, you want to be SURE you have enough time to fire. Roll through his attacks when he closes the distance and then roll away, charge up and fire when you can. Keep in mind that your stamina won’t start to recover for about 2 seconds after firing, so you can’t spam the weapon. .
As found on Youtube