Mythoclast is the weapon that we only get from one drop: Defeating Atheon on Hard.
The Vex are obviously mixed up in space/time theory, and there's plenty of solid proof from Grimoire etc that leads into the fact they are from our past, present, and future. In fact, the Gorgons and Oracles have the ability to erase us from existence and Atheon can transport us to either the future or the past.
The theory I read suggested that the return of Crota has already been seen in the future and the Vex cannot handle it (remember the Vex are so evil they hunt other evil, yet they never appear in places the Hive populates). In a last attempt, knowing already that a team of Guardians will defeat Atheon, they placed a "god-killing" weapon in his possession for them to gain. The DLC is all about stopping the resurrection of Crota, and the Mythoclast is a unique tool for completing this.
I stole this from another forum. Thought it was interesting.