Once he was one of the foreign constables chased a beast all the way to Yharnam. The constables became victims of the beast. The only survivor is Valtr who in turn devour the beast whole in vengeance for his former comrades thus gaining the name Valtr, beast eater.
This inhuman act, however, open his eyes and let him see Vermin, the source of man corruption. He then fouded The League whose mission is to crush the Vermin.
Found within the windmill shortcut directly after the Forbidden Woods lamp.
Valtr's questline is quite straightforward, all that is needed is to acquire and crush Vermin to progress it.
After crushing one Vermin, Valtr wil give the player's the "League Oath" gesture, and the League Staff.
After crushing a total of five Vermin however, Valtr will disappear permanently and leave the players the Master's Iron Helm as a gift, thus making the Hunter the true master of the League.
Beware as a Madara's Twin will attack the player. Killing him will drop the Madara's Whistle.
Click to see Dialogue (Contains Spoilers)
Ah, a new face are you? And an accomplished hunter... it would appear. I am Valtr. Master of The League, members of The League cleanse the streets of all the filth that's spread about during the hunt. Like any half-decent hunter ought to, you know? Haven't you seen enough of these wretched beasts, freakish slugs and mad doctors? Sentence these fiends to death, with the help of your League confederates! What do you say, why not join The League?
Joining or Not Joining
Yes... I suppose. I should not force your will, but you are a hunter and will see soon enough... that on a night of the hunt... no, on any night -- nothing out there, deserves to live.
Well then, you've seen enough out there? Fancy joining the ranks of The League? Yes as a hunter well should! Commit the start! Our own Caryll Rune, symbol of The League. The night brims with defiled scum, and is permeated by their rotten stench. Just think, now you're all set to hunt and kill to your hearts content - hunt in co-operation with your fellows, your league confederates.
Now, there is one thing you must know... by the oath of The League, those who bear its rune will see Vermin. Vermin writhe deep within all filth, and are the root of man's impurity. All Vermin are to be crushed. The League exists to expunge all Vermin, ridding us of any trace of human corruption. And so, until we are rid of all Vermin, you must continue to hunt and kill. This bloody fate is ours alone. Do not expect the world to grasp our work, but remember, the confederates will always have my blessing, and each-other, always.
Ahh, very good! You've crushed some Vermin. I'm the master of The League I can see it in your eyes. I'm pleased, this makes you a true fellow of The League. A confederate! Now, take this staff... a symbol of our oath, of our blood-drenched fate. You will be welcomed as a true confederate, go forth, with renewed vigour. In short time, you will see... How the mission takes hold of ones spirits.
Talking as a True Confederate
Hello, confederate. Has your stomach not turned, turned at the world of man? So wondrously wretched, and brimming with unsightly Vermin. Much like our very own mission.
Hello, confederate. Your eyes say all, you've crushed Vermin, time and time again. You've seen the filth that varnishes the world of man, yet, you are unbroken. You've the eyes of a hunter, you have blessed The League with your presence. This was my last, most pressing task.
My confederate. Promise me you'll crush all Vermin, to rid us of our impurities, for the sake of our fellows. All blood-stained hunters of The League.
Well, how quaint. You scoff at The League? Then you are yourself, a beast! A defiled mess of rotten flesh. When you are dead, your rank blood will curdle with Vermin!
Much like Eileen the Crow, Valtr is losing the ability to carry out his duty. As such, when the player shows themselves worthy, they inherit these character's duties.
The Master's Iron Helm mentions that Valtr has long ago lost the ability to see vermin, making him almost completely useless to The League. This might explain why he pass the leadership to one who deems worthy.
He is one-eyed.
Due to the fact that Valtr is a male character that offers you to join a covenant in which you assist other players in their respective worlds, and that he has helmet that resembles the shape of a bucket, it is more than likely that Valtr is in fact a reference to Solaire of Astora from the original Dark Souls. An interesting bit of information that seems to corroborate this is the vermin which are received at the end of co-op, they resemble the Sunlight Maggots that corrupted and drove Solaire mad, especially this small text that comes from its description: "Perhaps there is some mercy in the madness."
Valtr is voiced by Jason Pitt, the man who voiced the Head of Vengarl from Dark Souls 2.