The Pied Piper of Hamelin is the titular character of a legend from the town of Hamelin, Germany. The legend dates back to the Middle Ages, describing a piper, dressed in multicolored (pied) clothing, who was a rat-catcher hired by the town to lure rats away with his magic pipe.
The devil's spell
As is usually the case, true events over time have been repeatedly reinterpreted & embellished. It is believed that in the 16th-17th centuries when the legend was written down, it was already perceived as a parable of Satanic inspiration. In this version of the story, a devil in the guise of the Pied Piper led children away from the town, but, according to the Grimm brothers, who recorded this version of the legend, did not manage to kill them. On their way through the mountains, the children founded a town in Transylvania, where they later lived.
Ergotism
Ergotism is defined as poisoning by ergot, a fungus that parasites on rye ears & contains an alkaloid similar to LSD. Indeed, ergotism was widespread in the Middle Ages, especially among the urban & rural poor, who ate rye bread and were forced, especially in famine years, to grind into flour the grain together with the parasite that grew on it. Consumption of ergot causes depression, hallucinations & fear, among other things.
This was the basis for the theory that the "revenge of the Pied Piper" was in fact the result of mass psychosis when one person draws others with him and the crowd which has lost its sanity and sense of self-preservation is quite capable of falling into a dangerous or disastrous situation.
Gypsy theory
It is assumed that the children were lured away by colorfully dressed gypsies who managed to lead them away from the city with songs and dances. However, this theory does not have many supporters.
UFO
Another rather exotic theory is that the Pied Piper was a UFO astronaut, who for some unknown reason became interested in the children of Hamelin. The theory is purely speculative and has no confirmation.