After about 650 BC a people called the Celts lived in England. The Celts had priests called Druids. The Druids were very important in Celtic society. As well as being priests they were scholars, judges and advisers to the kings. The Celts were polytheists (they worshiped many gods and goddesses). They did not build temples but instead worshiped at natural sites such as groves of trees, springs, rivers and lakes. Sometimes the Celts sacrificed valuable goods by throwing them into lakes and rivers. In Celtic times the old Bronze Age practice of building barrows to bury the dead in died out. Instead people were interned in individual graves. They were still buried with grave goods showing the Celts had a strong belief in an afterlife. They believed that when you died your spirit went to a place called the Other World.
The Druids did not build Stonehenge. That is a historical myth. In fact that was built long before the Celtic Era. It is sometimes claimed that the Druids practices human sacrifice but is that true?
Julius Caesar (100-44 BC) conquered the Celts who lived in Gaul (modern France) and he led 2 expeditions to England. He wrote that the Druids sacrificed human beings by placing them inside a giant wickerwork and thatch effigy of a man then burning it. Caesar claimed that the Druids normally sacrificed criminals but if they could not find enough of them they used innocent people. However Caesar may have written that in order to justify his wars against the Celts (look how barbaric the Celts are they need us Romans to civilize them). In other words it may be propaganda.
Slightly later a Greek called Strabo (c.64 BC-24 AD) again claimed that Druids sacrificed human beings by placing them in giant effigies of men made of wickerwork and thatch and burning them. He also claimed they sacrificed people by impaling them or shooting arrows at them. However historians believe the Celts did not use the bow and arrow! So Strabo's writings are suspect.
The Romans strongly opposed the Druids. They had great social and political influence and the Romans probably saw the Druids as a threat). Therefore anything Greek-Roman writers say about the Druids is likely to be very biased and should be treated with caution. There is actually very little evidence of human sacrifice in Celtic Times. In 1984 the body of a man was found preserved in peat in Northwest England. He had been hit on the head and strangled and his throat was cut. Apparently he was the victim of a ritualistic killing in the 1st century AD. However there is no proof that the Druids killed him. We are not sure who killed this man or why. In summary it is possible the Druids practiced human sacrifice but it seems clear that if they did it was rare.
Another myth is that the modern Halloween custom of trick or treat is derived from a Druid custom. In reality Halloween customs evolved in the late 18th, 19th and early 20th centuries. There is no evidence that trick or treat has anything to do with the Druids! (See A History of Halloween)