This is quite possibly the most fun that I have had playing Mabinogi since I started playing the game almost 2 years ago. The chain slash class and skills are fun to use and are IMHO strong thanks to their powerful crowd control and "untargetability" frames. The knock-up and knock-back skills along with the distance-melee hybrid attacks will give your character a lot suitability. Everyone (new and old players) can learn how to use it and be effective at it. Lastly, it is more fun than the usual "normal attack + smash smash + block/counter" based combos and their derivatives. The flexibility makes me think me of a simplified version of the puppetry class!
Without revealing much of the story, at the end of the skill quest line there are more skills to unlock. One of them being Spinning slash. This post is a compilation small clips of the exact moments when and how I unlocked mine. I hope this post helps you confirm where the crystals do spawn.