Use the longbow as the base weapon, give it +2 to attack and damage with an extra 1d4 radiant damage tacked on, and the following feature:

The bow has 4 charges. Once per turn, when you make an attack with this longbow, you can choose to spend a charge to conjure an arrow of light. When you loose this arrow from this bow, a line of radiant energy that is 600 feet long and 5 feet wide fires from the bow. Each creature within this line must make a Dexterity saving throw (DC = 8 + Proficiency Bonus + Dexterity Modifier), taking radiant damage equal to the bow's normal damage (1d8 + 1d4 + Dexterity Modifier +2) on a failed save or half as much damage on a successful save. Fiends and undead have disadvantage on this saving throw. The bow regains 1d4 expended charges daily at dawn.

Once per day, you can allow the spirit of the angel within the bow to imbue you with its strength. For 1 minute, you gain the following benefits:
- a pair of spectral angelic wings emerge from your back. You gain a flying speed of 30 feet. If you already have a flying speed, your flying speed increases by 30 feet.
- Fiends and undead within 30 feet of you must make a Wisdom saving throw (DC 15) or become frightened of you for 1 minute. While frightened, they are vulnerable to any damage you deal to them. Creatures can repeat the saving throw at the end of their turn, ending the effect on a success.

For the cons, have it be that the bow will never attack a non-fiend/undead unless it is in self-defense (in other words the player can't be the aggressor). The player can still fire the bow, but it will lose its magical properties against these targets (i.e. doesn't have +2, doesn't deal 1d4 radiant damage, and can't use the light arrow). You could also have a personality change occur during the angelic possession, or maybe even a blatant takeover.