Zephyr is a unique Dwarven bow found in the Dwemer ruin of Arkngthamz during the quest "Lost to the Ages." It previously belonged to Katria, an adventurer who died in an earthquake while exploring the ruins.
Acquisition
During the quest "Lost to the Ages," as the Dragonborn and Katria make their way to the summit, they will come across a large chasm which Katria will mention she fell into and died. She also mentions that her bow is still sitting at the very edge of the log, overlooking the chasm.
To obtain the bow, the Dragonborn must carefully walk along the log and pick it up at the end of the log. The bow cannot be picked up from a distance by using the Telekinesis spell. Once it has been picked up, Katria will tell the Dragonborn to take good care of it for her.
Enchantment
Its innate enchantment grants it the ability to fire arrows 30% faster. Like all innate enchantments, it does not run out of charge and cannot be disenchanted or enchanted.
Smithing
It can be upgraded with a Dwarven metal ingot at a grindstone and also benefits from the Dwarven Smithing perk, which doubles the improvement.
Usefulness
Zephyr has the highest base damage per second of any archery weapon in the game, firing an arrow every 1.58 seconds, compared to the fastest non-unique bow in the game, the long bow, which fires an arrow every 1.81 seconds.
Early on and even at higher levels the Zephyr is a powerful weapon for archers. Able to deal a higher amount of damage per second than any other ranged weapon, Zephyr can theoretically kill faster than any other ranged weapon. However, it may be a poor choice at higher levels for archers that prefer sneak attacking, as the relatively low damage will not kill most enemies in one hit.
Since this bow relies on its high rate of fire rather than its base damage to bring down enemies, it can quickly use up arrows, especially at higher levels when enemies will take multiple shots to kill. The fact it cannot be disenchanted further limits its potential.