The Phoenix crossbow is a quest item, although it can be traded. It has the same stats as a regular crossbow, but weighs slightly less. It can be obtained and is used in the quest Shield of Arrav. The Phoenix crossbow has no requirements to wield, and can only fire bronze bolts.
The Phoenix crossbow is not often used, as the bronze crossbow requires the same Ranged level to equip, but gives a Ranged accuracy of +18, whilst the Phoenix crossbow only gives +6. However, the phoenix crossbow is essentially free.