The bow is originally owned by Karliah, but it can be obtained during the Thieves Guild questline. Because the bow is leveled, the magnitude of the enchantments, the base strength and value of the bow are contingent upon character level; meaning that with every level the Dragonborn has advanced when the bow is obtained, the bow does more damage, weighs more, and is worth more.
It is a reward given to the Dragonborn by Karliah, after they complete the Thieves Guild quest Blindsighted. The Nightingale bow can be upgraded with an ebony ingot and the Arcane Blacksmith perk, however it does not benefit from any smithing perks. This means that the weapon cannot be improved past flawless quality without boosting the Smithing skill over 100. This can be achieved by using enchanted items and/or blacksmithing potions. [Source: wikia]
You can download the papercraft model here: The Elder Scrolls V: Skyrim - Nightingale Bow Free Papercraft Download