Well since it's for the Celts/Druids here's some good information I got from
http://religioustolerance.org (a very good website I might add) about the Druid religion..
Afterlife: They believed that the dead were transported to the Otherworld by the God Bile (AKA Bel, Belenus). Life continued in this location much as it had before death. The ancient Druids believed that the soul was immortal. After the person died in the Otherworld, their soul reincarnates and lives again in another living entity -- either in a plant or the body of a human or other animal. After a person has learned enough at this level, they move on after death to a higher realm, which has its own Otherworld. This continues until the individual reaches the highest realm, the "Source." A Druidic visitor to this web site wrote: "All things are created from the Source, including the Gods. We are just sparks from its flame." At every birth, the Celts mourned the death of a person in the Otherworld which made the new birth possible.
Call it:
1) The Otherworld
2) Bilebith (Bile's world in Irish Gaelic)
3) The Source
4) Reincarnation
Or something similar to those.