You must complete the Grasp of Avarice dungeon to earn the weapon.
If you are actually looking for the Destiny 2 weapon guide, these videos show the exact steps to get it and its catalyst: Gjallarhorn-TENOKE.rar
The catalyst is obtained by finding three hidden chests guarded by Reaver Vandals within the dungeon. You must complete the Grasp of Avarice dungeon