When you play the wedding level "Party Garden", there will be a sign telling you to check out the lower left corner of each Side Quest level. So, do that and have some fun :)

This short guide is about another thing though: Another sign will ask you what the stone in the level "Scenic Route Road" says. And surely enough, near the end of the level you find this stone:



They look a lot like Nordic runes, don't they? Now, let's take a Runic alphabet and use some interpretation skills to decipher the runes:


Source: https://en.wikipedia.org/wiki/Runes#/media/File:Medeltida_runor.svg

The resulting words are: "lightning life shield fire"

Now try that in the game. Fireworks! And the achievement "Go out with a bang" as well!