Where do start a monster etc

Geriden

hay where in the game do you go to start this Monster pvp thing i cant find it

and what lvl does it require to start it anyone know ?

Krogg

Once any of your characters reaches 10th level, you are granted access to play 5 monsters in Monster Play.

To begin you will visit a shrine to the Dark Lord in a back alley in Bree-town (can be found using map filter), and then you can basically choose any of the 5 Monsters to play, name them all, and rank them up, etc.