Xholankha

The Old One, Xholankha, the Lost, is rarely invoked, for he is believed to be lost to the Lizardmen and therefore unable to hear their prayers. Some Slann Mage-Priests believe that Xholankha engaged upon an all-but impossible mission at the very beginning of time, from which he may, if successful, one day return.

Source
pg. 51
 * Warhammer Armies: Lizardmen (7th Edition)