Baishens are creatures that are often seen as fallen gods in the Catologian Faith. They were Gods who had sought power during the Chaos War and refused to obey the Tetrarch Gods and Their new Council of Gods. They were defeated and locked up. Since then they try to regain their old powers and status. To reach that goal they have to overthrow the Council, and for that they are seen as sworn enemies of the Council Gods.
They are not necessarily bad creatures, but overthrowing the Council or another celestial war could bring heavily damage to Time and the Universe. For that reason they are also seen by the clergy as a danger to humanity and should be fought by all means possible.
The best known Baishen is Haruk, one of the eight First Gods, who defeated the other First Gods - with exception of Sakat - and was close to victory during the Chaos War. He was so strong that even the combined power of the Tetrarchy - while connected to the Source of Life - could not destroy him. Instead he was locked for eternity beneath Mount Temoe.