Lesson Learned

Lesson Learned will upgrade a random card in your deck if it is used to kill an enemy. After being used it exhausts, whether it has killed an enemy or not.
The kills must be considered to upgrade a card. Not all enemies count as Fatal kills (exceptions detailed below).
==Interactions==
 
*Lesson Learned is capable of upgrading itself.
* can be used to return Lesson Learned to your deck, allowing for more than one use or kill per battle.
* gives a 50% chance that Lesson Learned will not exhaust on use.
 
*Minions do not count as kills. These are recognized by the icon. For a list of minion enemies, see the Minion page. 
*If fighting Darklings, reducing the HP of one of them to 0 does not count as a Fatal kill unless it was the final enemy from the batch.
*Defeating the first phase of the Awakened One does not count as a Fatal kill.
*The card it upgrades will not be upgraded during the fight itself, and only from the next fight onward.