Dragon's Breath

Buy it from the Exotic Archive at the Monument to Lost Lights, in the Tower between the vaults, filed under the year it shipped in. An Exotic Cipher and a pile of Glimmer, on any character, with no quest and no drop chance in the way.

It was a season pass reward, which is why the Archive is where it lives now.

Exotic Ciphers are the only part worth planning around. Xûr sells one a week, and the seasonal challenges hand them out several times a season. If you’ve been ignoring them you probably have one already.

The catalyst

Banshee-44 has the quest. It wants kills with the launcher and calibration data, which accrues from running activities with it equipped rather than from anything specific. The honest way to finish it is to use it, which any Solar build was going to do anyway.