Advanced Beings and Adventures in the Inner Planes (EAFW)

When human clerics or druids advance beyond 20th level, they must choose whether or not to remain human priests. If they decided to remain as they are, they continue to advance and gain the powers that are granted as levels are attained.
The other choice, and one open only to clerics and druids who are also psionicists, is to transform into advanced beings. Clerics become cleric elementals, and druids become spirits of the land.
When these fantastic transformations occur, the priests are often summoned to the elemental planes to fight pitched battles with the other spheres.