As far as I know, there are more than eight Daoist immortals so I’m not sure what sets the eight, in the group known as The Eight Immortals (Bāxiān 八仙), apart from the rest.
Each immortal has an instrument or implement onto which the immortals may transfer his/her power. These instruments are known as the Covert Eight Immortals (暗八仙).
The exact identities of the Eight Immortals are unknown, but some of them are associated with actual historical figures born during the Tang and Song dynasties. Apparently they reside on a group of five mythical islands in the Bohai Sea, namely Mount Penglai (蓬莱) Island, Fāngzhàng (方丈), Yíngzhōu (瀛州), Dàiyú (岱輿), and Yuánjiāo (員嬌).