Evolve unlocks at league level 18 and it allows to trade a bunch of cards for a few of higher rarity. No gold is needed for this, but you need to have unlocked the higher rarity card to proceed.
A guild bonus named guild bonus effect reduces the number of cards needed to evolve, and some events will double the amount of higher-rarities (limited to certain families only).
|Common||50 to 5||500 to 5||/|
|Rare||/||50 to 5||200 to 1|
|Epic||/||/||20 to 1|