Achlyophage Symbiote is an Exotic Hunter Helmet which grants your Golden Gun an additional shot (from 3 to 4).
Excerpt from "'Should I Buy It?' – Patience and Time & Other Exotics"
Achlyophage Symbiote is one of three Gunslinger-specific helms; the others are the speckled Young Ahamkara's Spine (Gauntlets). The main feature of Achlyophage is an extra Golden Gun shot, upped from 3 to 4.
If you're a Gunslinger and don't own one, absolutely. It's a strong pick in PVP and PVE, as an extra shot often directly translates to kills / Orbs of Light. In PVP, it takes two shots to destroy a Defender's shield; the two remaining shots can vaporize multiple exposed Guardians. In revival PVP gametypes & Nightfalls, we recommend Crest of Alpha Lupi instead. Otherwise, Achlyophage Symbiote is our preferred day-to-day Exotic.
Since Symbiote boosts your Golden Gun damage, it's being compared with the new Celestial Nighthawk. Ultimately, it comes down to if you're willing/able to patiently line-up PVE targets. Nighthawk wins if you can make its 1 shot count. Otherwise, Symbiote's four-shot versatility still wins out for PVE – and unquestionably beats Nighthawk in everyday PVP.