A new race – the Order of Knights of Elrath.

You could try making those as well. Hippogriffs are better than eagles.

Eagles don't need to be changed; something else needs to be sacrificed to introduce hippogriffs. Here's some information about hippogriffs:

Hippogriff - ability "Precise Strike" (almost the same as "Heavenly Strike," but it attacks only one enemy unit and deals more damage),
Hippogriff King - passive ability "Aura of Courage" (cannot have morale lowered in battle), 25% chance to resist any magic, even Light magic, and 40% chance to deal double damage.
Hippogriff Spellcaster - when attacking an enemy, it always casts a Dark magic spell on them and uses "Fire Tornado" once per battle (throws fire arrows in different directions, dealing 50% of the creature's normal damage; it also hits its own units).