king Vendrick was once a proud ruler of drangleic on his way to becoming a true monarch(similar to lord Gwyn), until his soul was taken by queen Nashandra, who was one of the shards of the soul Manus once possessed. he is found in a crypt, guarded by many knights and spirits, as well as one of his strongest knights, velstadt.
King Vendrick will swing at you through the whole fight, slowly, granted, but it can remove a large sum of your health bar and possibly even insta-kill the player.
Occasionally, if you go back into the area after death, he will throw a hex at the player, this can do much the same to the player as any of his other attacks.
In order to begin the fight, attack him with bleed weaponry (anything imbued or coated with a bleed substance), as it is the only way to phase him.
With the giant souls in your inventory, you deal more damage than you would without them. The maximum you can have for damage scaling is 5.
attacking the back
Vendrick seems to always swing at the player if the player is in his field of vision, so if you stay behind him (taking occaisional breaks as if you don't he backsteps out of the way) you can easily kill him.