How is benzene prepared from acetylene?
Walter Reppe developed a process for the "safe" trimerization of acetylene to benzene and cyclooctatetraene. Although completely superseded by more modern alternatives, this process was used before/during WW2 because starting from only coal, calcium carbonate and electricity one could synthetise calcium acetylide and therefore acetylene, having no need for oil derivatives, scarce during war.

The mechanism (in simplified terms - hold onto the stones!) is that on the surface of the catalyst (iron, copper or nickel depending on the variation of the process) there would be formation of transient "clusters" of three or four molecules of Fe/Cu/Ni-acetilyde. That close arrangement facilitates one of the electrons of the triple bond of the acetylide "attacking" one of the carbon atoms of one of the nearest acetylide neighbours (again, simplifications). That molecule with an excess of one electron would in turn atack its other in vicinity and finally this one would attack the first one (that became deficient in the first step). 

Since the aromatic structure of benzene is rather stable compared to ethyne, and the product is 1/3 (or 1/4) the number of moles of reagents, the formation of product is favored under pressure.

Some sources:
Walter Reppe and the chemistry he developed: https://en.wikipedia.org/wiki/Walter_Reppe
Trimerization: https://en.wikipedia.org/wiki/Trimer_(chemistry)