We all know that the only way to get the Necrochasm it to defeat Crota on hard. If you want the Crux of Crota you have to treat the Crux of Crota not as a material but as a weapon. What i mean is it is similar to getting the Vex Mythoclast you have to finish the raid a certain number of times to achieve it. Even though that the game is based off of random drops there are certain way to have a higher probobility of having something drop. So what im saying is if you want to raise your chances of getting the Crux of Crota Complete the Hard Raid 2 or 3 times a week on the same character.