Karthakh was a Veltrochni assassin. He and his partner Sha'ol 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.