Shovel Knight: Treasure Trove

This achievement is unlocked by gaining all arcana, which are Plague Knight's version of relics.

This is done in two parts.

1. Generally within the first half of the level, there will be a somewhat long bonus area that is not present in Shovel of Hope. At the end of this area, there will be a bronze chest containing a Shovel Knight relic (flare wand, phase locket etc.).
2. After obtaining this relic, you will then need to find Chester's blue relic chest, which is always later in the level than the Shovel Knight relic chest, and in the same place as the blue relic chest in Shovel of Hope. Chester will prompt you to trade in the relic you obtained before, and get the arcana for the level in return.

You will need to do this for every Order of No Quarter stage to get the achievement.

Here is a 100% playlist where you can filter through the videos for the appropriate relic chests, but you shouldn't have too much trouble finding these areas naturally.