The Changeling is in the Irish legends, a lure put in place by fairies to replace a newborn that they have just removed by malice. More specifically, it is a deception, allowing the parents not to be alarmed, as they are persuaded to have their child with them.
The Changelings are traps found in Irish and Scottish writings. It is said that fairies, out of malice and curiosity, would not hesitate to take babies from men, in order to have fun. In substitution, the fairies would then place a Changeling in the place of the abducted newborn baby.
The shape of a Changeling can change from one story to another, but most of the time, a Changeling takes the shape of a fairy baby, or an enchanted piece of wood. The spell then wants this substitution to make the parents totally deluded, as they are persuaded to have their child next to them.
However, there is a technique for unmasking them: it would be enough to arouse the Changeling’s curiosity for it to reveal the whole truth to you!
For parents who have unmasked the subterfuge, it is then possible to catch the Changeling, and to summon it to return their child to them. Generally speaking, fairies always return newborns to their parents: they would only have taken them away for fun.