Sha'ol was a S'Raph Tzun assassin. He and his partner Karthakh were hired by Mr Zimmerman to kill the Sixth Doctor. However, when they appeared in the TARDIS they were out-of-phase and the Doctor ejected them.
They survived and returned to Vandor Prime. They stole the TARDIS to get the Doctor to come to them. Niccolo Mandell offered them a bonus if they would delay their contract until after the Doctor completed the job on which Mandell had sent him, and they agreed.
They later saw the Doctor apparently electrocuted while fighting one of Mandell's agents. Reporting this to Zimmerman, he paid them for their completed contract. (PROSE: Mission: Impractical)