What happens in the reaction of ammonium dichromate and mercury(II) thiocyanate?
This is a combination of two reactions.  The first is the ammonium dichromate vocano and the second is called Pharaoh's snakes.

The volcano

The reaction occurring here is: $$\ce{(NH4)2Cr2O7(s) -&gt; Cr2O3(s) + N2(g) + 4 H2O(g)}$$  The reference above provides several pictures and a description of the chemistry.

Pharaoh's snakes

Again, the reference above describes what is happening, so only a summary of the chemical reactions is given here:
$$\ce{2 Hg(SCN)2-&gt;2 HgS + CS2 + C3N4}$$ $$\ce{CS2 + 3O2 -&gt; CO2 + 2SO2}$$ $$\ce{2C3N4-&gt;3(CN)2 + N2}$$ $$\ce{HgS + O2-&gt;Hg + SO2}$$

Pharaoh's snakes is a rather hazardous demonstration and should not be performed without the proper ventilation, protective gear and waste disposal equipment.