where is the labirint where lazarus and the prince are ?

When you reach level 15, look around for the Lazarus' staff (it's on some sort of a weapon rack surrounded by torches, you can't miss it). Take it to Deckard Cain in the town (the one who identifies your equipment). He'll take the staff and reveal an interesting part of the storyline. Next, go back to the level 15 and look for the pentagram on the floor (shouldn't be hard to find). There should be a red portal opened next to it. Enter it to reach Lazarus.