Well, the official book universe explanation is that words have power when spoken by the gods, so when they speak an untruth, they can unintentionally change the nature of something. That idea is heavily inspired by Diane Duanes’ Young Wizard series. But I included it in my universe because
1. the gods in my universe represent deities across cultures and mythologies. Hades isn’t *just* Hades, he’s the Hades equivalent in all mythologies. Some got more details right than others.So the rules of my universe needed to come from more than one culture. The not being capable of lying thing is borrowed from fairy lore.
2. There were a several instances in Greek mythology of gods being quite literally bound by their word (swearing by the Styx for instance, or the entire concept of Xenia, which the gods were bound to obey.) I needed to include aspects from those myths, but applying them randomly made no sense/would be, believe it or not, more complicated to maintain. The gods needed limits, firm rules, for the laws of my universe to feel real. Thanatos isn’t likely to get Persephone to unintentionally swear by the Styx, but to accidentally forget and lie? That’s a major limitation, and one that makes the magic systems of the gods feel more real.
Either way, it adds a whole new level to editing that I’m sure the folks over at Belle Books just love :D.