Lady Leech of Cornwall

Then the king [Mark] let send after all manner of leeches and surgeons, both unto men and women, and there was none that would behote [Tristram] the life. Then came there a lady that was a right wise lady, and she said plainly unto King Mark, and to Sir Tristram ... that he should never be whole but if Sir Tristram went in the same country that the venom [of Marhaus' spearhead] came from.

One can assume from the context that she was a leech or surgeon; she may, of course, have been an herb-woman or even one who simply knew how to speak out with authority. Mark and Tristram took her advice seriously and followed it, to Tristram’s healing.