Caesar, De Bello Gallico 6.16
Sacrificial Customs of the Gauls
The entire nation of the Gauls is very given to religious scruples, and for this reason, those who are affected by grave diseases, and who take part in battle and in peril, either sacrifice men as victims or vow to sacrifice them, and they use druids as the performers of these sacrifices, because they believe that, unless for the life of a man, the life of a man is returned, the power of the immortal gods is not able to be appeased, and they have instituted sacrifices of this kind for the public. Others have images great in size, the limbs of which, interwoven with twigs, they fill with living humans; the men, with these having been set aflame, perish. The punishments for those apprehended in conspiracy or in thievery or in other crime are thought to be most pleasing to the immortal gods; but, when abundance of this kind fails, they even defer to the punishment of the innocent.