MagisterCultuum
Great Sage
Darkheart is right. Level 6+ living Sidar heroes can cast wane, becoming shades. Unliving heroes (like Sphener, Mardero, and Saverous) cannot become shades. As the Sidar, you could get 9 free shades from religious heroes (Bambur, Arthendain, Kithra Kyriel, Yvain the Woodelf, Valin Phanuel, Rosier the Fallen, Chalis Ashtrakien, Hemah, and Gibbon Goetia). Also note that shades keep their promotions, so a caster (archmage, summoner, hemah, Yvain, Gibbon) can still use its spells after becoming a shade. This means that if you are willing to pass up some free great specialists that you can have the strongest magical army in the world, with unlimited equivalents of national unitsplus extra heroes. Of course, shades cannot get any new promotions, so you must decide carefully when to use Wane.
Gilden is the Ljosalfar hero though, so I guess you would need to capture him with an inquisitor first.
Gilden is the Ljosalfar hero though, so I guess you would need to capture him with an inquisitor first.