Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word: You look up to see a crazy illusion. You can't believe it's real. Master Roshi, from Dragonball, is standing in front of you (you thought he was a cartoon!). Then, he splits into a million versions of himself, creating a mob of Roshis (まぼろし). This mob goes on to train many people to fight.