This return the custom field value from the post meta
// returns the value of health-gain
$health_gain = get_post_meta( 11, 'health-gain', true);
$herotip.= '<a href="http://stormable.com/heroes/illidan/">Illidan</a>
This is where I would like to pull in custom field data from the post illidan
For example '. $health_gain;
take a look on this link get_post_meta(), Implement it into your shortcode function where ever you want.
I Suggest you to take a look on this article http://wp.smashingmagazine.com/2012/05/01/wordpress-shortcodes-complete-guide/, tells you the more robust and right way to create a shortcode.