FateStay Night: Raven's Lament

In the F/SN [UBW] universe, Rin Accidentally summons.. an Assassin Class? This Assassin from another Universe uses his skills from his past life's. What's his goal, what's his ambition, is there something lurking in the shadow's? Either way those who oppose this new player of the game that is the Holy Grail war, will Repent, For this, is a Raven's Lament. M to be safe