From Destinypedia, the Destiny wiki
Insidious is a Legendary Pulse Rifle that can be acquired from the second and final encounters in the Vow of the Disciple raid. It can also be crafted once five Deepsight Insidious' have been obtained and completed.
An Adept version of this weapon, that is able to equip Adept mods, receives extra stats from masterworking and has a slightly different appearance, can be acquired by completing encounter challenges in the Master difficulty version of the Vow of the Disciple raid.
List of appearances