Ngueak, and the nāga which have been "tamed" by Buddhism, were believed to bring rains, or change shape, and nāga in particular were believed to be protection spirits which inhabited the cities of Vientiane and Luang Prabang in Lan Xang.
Phi were believed to influence natural phenomena including human illness and thus the baci became an important part of Lao identity and religious health over the millennia.