How many stats does a star give, by dragon rarity?

The stat bonus per star depends on the dragon's rarity. Based on the rank upgrade offers (1★→2★):

• Legendary: +20% to all stats per star (e.g. Syrax)
• Epic: +15% to all stats (e.g. Shadowsong, Daemoros)
• Rare: unconfirmed, probably +10% (check in game)

The strategic consequence: for the same Relics, a star pays more on a Legendary than on an Epic or a Rare. One more reason to concentrate your best Relics on your key Legendaries rather than sprinkling them everywhere. And since Star Rank is permanent, that +20% follows you Reign after Reign.