The Phantom Slasher is Lyrra (Crazy, but it all makes sense once explained in-game... just keep in mind that it has to do with the Landorin Massacre). So, I guess that's what he means by "when not a shadow".
If this is true and you just spoiled it for everybody, I am going to personally skin you, dip you in vinegar, light you on fire and watch you burn for betraying our trust, and most importantly Lun's trust. And even if you're just BSing us, I'll still skin you, but use salt instead. THOU DOST NOT LIE WITH SOMETHING OF THIS MAGNITUDE!