First you must have the husk of pit. Which has a very low drop rate on the fist of Crota story mission. Kill the knight that spawns just after you enter the complex. Repeat that until it drops. Once it drops upgrade it to the necro. After that get your crux from Crota raid hard mode (I'm not sure if it drops in easy mode) and your done.

Necrochasm? No it's not possible. It's a raid weapon.

You can only get it through the raid (crux of crota)