A baptism was a dark ritual undertaken by the Nightsisters of Dathomir, using the dark magicks, that would result in an individual being proclaimed a true Nightsister. During the Clone Wars, baptisms by the Nightsister clans would take place in an oily subterranean lake beneath the Nightsister lair.
The ritual would take place beneath the Nightsister lair of Dathomir, and required the Mother of the Nightsisters, during the Clone Wars that title was held by Talzin, as well as several regular Nightsisters. The Mother would question the would-be Nightsister on their loyalty to their clan, and if the prospective sister was deemed fit, they would be dunked into the waters of the lake beneath the Nightsister lair. The Nightsister would also be wrapped in magical ichor by the Mother, and lifted from the lake. For a moment, the individual's eyes would roll back into their heads, and then return to their natural state, signifying the beginning of their life as a Nightsister.
After the baptism, the Nightsisters would organize a feast to celebrate the arrival of their new Sister.
Ventress's baptism
One such baptism took place during the Clone Wars, when Mother Talzin baptized Asajj Ventress, who came to Dathomir in the hopes of being re-initiated into her clan after being exiled from it for a long time, as an apprentice to Count Dooku.