It is completely optional and very difficult. I gives you a soul of a giant which can be held to reduce King Vendricks defense and a single petrified dragon bone. The other 4 giant souls should be sufficient to kill Vendrick (it was for me but it was still difficult).
The boss is super hard but he can be beaten:
-Two hand the weapon as any attack will be a one hit kill anyway.
-Dont summon anyone you dont know as theyll die instantly.
-Bait him into his forward fire breath and attack his legs. Immediately run away if he starts to flap his wings. Rinse and Repeat.
-It totally sucks but its doable. Took my 10 minutes with a two hand +10 lightning great sword.
if you talked to him and got the item from him, you can go to where you fought Freja and enter his memory to get his soul. But, he does drop one of 5 giant's soul when you kill him, which lower's Vendrick's defense if you decide to fight him, and is almost necessary to kill him
You can also just stay underneath him right next to his back foot, hit it a few times and he'll try to stomp you, dodge to the other foot, he'll also occasionally raise his front feet and breath fire between his legs, just run out from under him when you see him do this and then back under him when hes done. That's how i did it and he never did his flying instakill fire breathing attack
The Bird of Hermes is my name, eating my wings to make me tame