Tony Martin was great in Sabbath. I think he has lost a bit of his voice recently though.
Him leaving after Tyr was more about Dio. They had the chance to reunite with Dio and they unceremoniously dumped Tony. Then they had to be nice to him again to get him back for Cross Purposes. Cross Purposes was a great album, with Bobby Rondinelli (another ex-Rainbow member) on drums. Cozy came back for Forbidden but that was terrible due to the production by Ernie C from Body Count (and the track with Ice T as guest), which was more a case of Tony and Geezer trying to be 'relevant' in 1995 rather than the fault of Tony Martin or Cozy Powell.
Tony was brought in after a disastrous drug-addled marathon recording session for Eternal Idol on Monserrat. Various issues happened, resulting in Ray Gillen being let go as vocalist and Tony Martin hired. Some of the stuff was re-recorded (mainly the vocals) but the drums by Eric Singer were kept from the first session. Not sure about the bass. Dan Spitz was definitely on bass for the Monserrat experience. Eric had left by then but they kept the drum recordings. This was also around the time that they received a

in a box from Sharon Osbourne. She had a habit of doing that. She claims that she got the kids to do it but Tony always had this image in his mind of Sharon squatting over the box. Not sure how it got through mail. Maybe she used a courier. I hope they charged her extra.