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 Self-perfection reduces the number of cards needed to evolve, and some Mini-Event will double the amount of higher-rarities (limited to certain families only).

Conversion ratesEdit

Cards can only be evolved from their current rarity to the next highest rarity. Brackets mean virtual conversion rates, that are listed only for math's sake

Rare Epic Legendary
Common 50 to 5 (500 to 5) (2,000 to 1)
Rare - 50 to 5 (200 to 1)
Epic - - 20 to 1

Evolve chartEdit

Here are some charts with all the possible evolutions, updated to Housekeeping event (2019-04-19). The sequence the cards are listed is roughly the same in which they appear on SQUAD tab.

Evolve Chart - Chaos

Evolve chart for Chaos units, updated to Dead By Bed event (05/April/2019)

Evolve Chart - Nature

Evolve chart for Nature units, updated to Protein Milkshake (22/March/2019)

Evolve Chart - Order

Evolve chart for Order units, updated to Housekeeping event (19/April/2019)