OpenAI CEO Sam Altman sought to turn the tables on Elon Musk in their legal battle, testifying that the tycoon abandoned the group they co-founded and worked to undermine its success.

Altman’s appearance in court Tuesday marked the highest-stakes moment yet in the trial over Musk’s lawsuit, which centers on allegations that the OpenAI chief led a deceitful effort to “steal” the charity that he and Musk co-founded in 2015. Altman faced repeated attacks on his honesty during cross examination as he tried to defend his leadership of OpenAI and his efforts to fulfill its original mission of ensuring that advanced AI benefits humanity.