Although Lachesta is indeed a deity of Warj'Arune, his existence and mysterious whereabouts are major plot-points of our ongoing story.
There are no public Clerics of Lachesta, or Blackguards, for any who would claim such things openly would be set upon most quickly by the Knights of the Last Stand. That is not to say that it isn't possible to become a Cleric or Blackguard of Lachesta, but rather that you cannot start as one. You will have to find a means of accomplishing this feat in game.
Only Minotaur PCs may start with Lachesta as their patron deity, though they are by no means required to, and they may also begin the game as a Cleric of Lachesta. While, other races have recorded history to draw upon, only the Minotaurs have the direct connection to their Abyssal forefathers that allows them to begin the game with an innate faith in their dark creator.
Remember, the name "Lachesta", and his story, are considered OOC knowledge, unless you are a member of the Knights of the Last Stand, a Minotaur, or have spoken with one regarding these issues, or have found this knowledge by some other "in-game" means.