Second Generation Slann use up an extra Hero choice, as well as a Rare Unit choice (so they take up a Lord choice, a Hero choice and a Rare Unit choice).
Every spell cast by the Slann may have an extra dice added to it after the dice have been rolled. This extra dice does not come from the Slann player's pool of Power dice, and is in affect a 'free' dice. This extra dice may cause Irresistible Force or Miscasts as normal. In addition, the Slann has +1 Wound (remember this is cumulative, so a Second Generation Slann has 2 more Wounds than a Fourth or Fifth Generation Slann).
However, as a Slann of the Second Generation is so ancient and revered, if he is killed in battle then the Lizardmen's opponent gains an extra 100 Victory Points at the end of the battle (remember this is cumulative, so the opponent gets an additional 200 Victory Points total, including the 100 for Third Generation). If the scenario does not use Victory Points then the demise of the Slann has no additional effect.