Apparently this video is a reaction of $\ce{NH4Cr2O7}$ with $\ce{HgSCN}$.

The title of the video suggests the reaction opens a portal to hell, but I am pretty sure that is not the case.

What exactly goes on in the reaction?

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    $\begingroup$ In short, this is a mash-up of two well-known experiments: Pharaoh's serpent and Vesuvian fire. The two compounds do not really react with each other (well, at least not much); instead, each reacts on its own. $\endgroup$ – Ivan Neretin Feb 25 '16 at 16:57

I can't comment on the metaphysical aspects ;)

This experiment is known as Pharaoh's snakes, and it is indeed impressive.

Upon heating, mercury thiocyanate decomposes to yield mercury vapour, carbon sulfide, and dicyan (ethanedinitrile):

$$\ce{2Hg(SCN)2 ->[\Delta] 2 Hg + 2CS2 + (CN)2}$$

As a result, the solid starting turns to a foam-like mass.

The effect is amplified due to the presence of ammonium dichromate, which oxidizes the carbon disulfide to sulfur dioxide and carbon dioxide.

The experimenter is apperently aware that quite a lot of rather harmful components are released and therefore has decided to run the show in the fumehood and closes the window immediately after igniting the mixture.

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    $\begingroup$ None of those products looks nice, but the cyanogen may just open a one-way portal to hell for some. $\endgroup$ – SendersReagent Feb 25 '16 at 22:20

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