To become invulnerable:

 1. Take a stone from the inventory and make it your cursor.
 2. Press [LeftShift], then [F9]. If you do this correctly, Drake's
    hit point will never drop below 1. The cheat doesn't affect
    anybody else in your party and needs to be reset each time you
    play.

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 Check out the file muffin.txt from the Stonekeep CD if you are hungry...