Rite of Emergence

From Memory Alpha, the free Star Trek reference.

The Rite of Emergence is a Trill rite that is performed by a joined Trill to bring forth the personality and memories of a previous host. This personality can then be interacted with by the current host.

This rite was used in 2375 by Ezri Dax to bring forth Joran Dax to help her investigate a number of mysterious deaths on board Deep Space 9. (DS9: "Field of Fire")