Precursor weapons are level 80 exotic weapons used to craft legendary weapons at the Mystic Forge. Precursor weapons are obtained by the same methods that any named equipment can be obtained, but have a much lower drop rate.
Precursor weapons can be obtained from almost any item source. This includes loot, chests, containers, reward chests, etc. that can be characterized by the possibility of receiving a weapon. The possibility of receiving a precursor weapon is not dependent on level or area, as players have reported receiving precursors on low-level characters in low-level areas as well as high-level character in high-level areas. To note, only high level players (62 - 80) can receive exotic quality weapons and may be more likely to receive precursor weapons from loot as loot often scales with level.
A precursor can also drop from any chest or container that drops weapons. Chests include jumping puzzle chests, dungeon chests, boss chests, and so on. Bonus Chests from world boss events have a greater chance to drop exotic or precursor weapons than other sources, being introduced in the 26 February 2013 update.
The Mystic Forge is a primary source for precursor weapons due to the refinement that allows players to mix together equipment for a chance at better quality equipment. Players have a chance to receive a precursor weapon when combining four rare or exotic weapons with an average level of 75 or higher. Using four weapons of the same weapon type guarantees receiving a weapon with that weapon type. For example, players can combine one level 60 rare weapon and three level 80 weapons. Player research into the chance of receiving a precursor when combining four exotic weapons of the same type estimates the probability at 0.79% with 2901 trials.
Precursor weapons can also be crafted. The Heart of Thorns expansion added a collection-based precursor scavenger hunt for existing and new legendaries that have yet to be added to the game. Players will be able to acquire one precursor per collection per account, these collections will be unlocked by spending mastery points. This new precursor acquisition system will allow players to unlock all legendaries. Heart of Thorns legendary precursor weapons are account bound (and thus cannot be traded), but precursors for the original legendary weapons can be sold to other players.