Once you have ascended a Troop to Mythic, raised it to level 20 and unlocked all 3 Traits, you can enhance your Troop even further by Awarding Medals to it.
You can find the Medal screen by opening the Troop Menu, selecting a fully upgraded Troop, select upgrade and then select Medals.
To enhance a Troop with Medals, it must be fully upgraded (Mythic rarity, level 20 and 3 Traits unlocked).
Once in the Medals screen, on the left you can see all the Medals you own and the % chance that they will be successfully sacrificed to your Troop.
You can attempt sacrificing up to 3 Medals at a time. The Medals with the best chance are Medals of your Troop's Base Rarity or higher. To make it easier to see your Troop's Base Rarity, it is written at the very bottom of the Medals screen.
Awarding Medals costs Souls and you will sacrifice your Medals and Souls regardless of whether the attempt is successful or unsuccessful.
There are 3 Levels of Enhancement: Bronze, Silver and Gold. The higher the Enhancement Level of a Troop, the harder it will be to enhance to the next Level.
Enhanced Troops gain a special flair on their card as seen below:
Enhancements provide Skill Points according to the Troop's Role and number of Enhancements. You can see which Skill Points each enhancement level will give a Troop by opening the Troop upgrade screen on the Troop, selecting Medals and then selecting their Troop Role icon above the medals on the right side of the screen.