Take the way to Mistwood PQ and go est to the edge of the cliff. Smoothly slide on the steep rock till you reach the water below. Go towards the round lake a bit north, watching out for the deadly guards of the order warcamp. You can avoid it by going round on the leftmost side of the warcamp. You need to swim in the round lake till you reach that small beach with the tainted cristal. For destro it won't unlock any ToK titles, but you still need to right click it. Once clicked, jump on the left side rock and enter the cave on the left.
Notes: before you right click the tainted cristal, the cave will be a normal one, with some low level lizards inside. When you activate the tainted cristal, it transforms into the Lair and you'll find the boss inside.