Valkyr (pronounced val - keer) is the fictional megadrug from the video game Max Payne. Valkyr was secretly manufactured by the monopolitic Aesir Corporation and distributed to just about everyone. It seemed to make the user as belligerent and insane as a user of PCP, only moreso; its addictive properties seemed to rival that of heroin, although no description of euphoria or other highs was forthcoming in the game's dialogue.
(Minor spoilers ahead.)
We learn from Max's own trip on the stuff that when a user was using, nothing seemed real and normal phrases wouldn't make the slightest bit of sense, and visual properties would take on the effects of LSD-like near-death experiences.
Valkyr itself is a green, gelatinous liquid that is dosed by injection. When Max encountered junkies in the game, they'd always be crouched over in some hovel bathroom or something, screaming and shooting at you with Desert Eagle or Beretta 92F handguns. They were usually the easiest enemies to kill -- usually one shot from any gun did the trick.
The drug was first developed by Aesir CEO Nicole Horne, while she was under the employ of the United States Army, at an underground research facility in upstate New York. It was originally intended to be a mind control drug for use in chemical warfare. Once the project was canceled, Horne continued to work on the drug and eventually perfected it, and then spread it across New York City through underworld intermediaries. The objective of Max Payne is, essentially, to put a stop to this. | 
