Sendell's ball is a magic ball that allows Sendell to comunicate with the one who has it.
To get Sendell's ball, Twinsen must go to the secret room in the sewers of Citadel Island that is opened with the pyramid shaped key. Once there, he needs to use the ring of lightning to break the sphere that protects the ball. After getting it, Twinsen inmediatly gains the red level of magic.
"Thanks to Sendell's Ball, you just reached your third level of magic: red ball. With its help, Sendell can come and talk to you in case of danger."