In Underworld: Blood Wars (2016), in the middle of the film, Selene is killed by Marius. But in the end Selene was alive again. So the question is, how did she come back to life?
The northern vampire clan she was visiting, were known to dable with death to further their enlightenment. We see a female vampire covering the face of another vampire submersed in water, who she says was dead. Although dead, she clarifies she is on the other side, but will come back, and the whole experience is treated as a spiritual voyage. She also says she did it herself a bunch of times, and the secret is in the water.
When fighting Marius, she conscientiously throws herself under the ice. Marius thinks nothing of it, and leaves her for dead, but the Northern vampire clan revives her, like they did so many others before. When Selene comes back to life, she is empowered by her journey like the northern vampires, and this is symbolized by her hair tips going white.