The acid flask is a cosmetic override available from the Death Lotus Training event, by completing the death lotus training scroll tasks once and claiming acid flasks from the prestige interface. At one point, they could be purchased as "Death Lotus Smoke Bomb overrides" from the Oddments Store for 800 oddments, but they are no longer obtainable this way. There is also an off-hand variant named "Death Lotus Off-hand Acid Flask."
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