Cicadas are a passive, very hard-to-spot mob found only in Elysian Fields. They are the sole source of Repellent in the Erebus and as such are necessary to craft anything requiring it (e.g., Water Repellent for Water Striders, Varnished Planks for a Composter, etc.).
When a player gets within ~5 blocks of one, they emit a ghastly noise, pushing the player back and inflicting 7 seconds of Blindness and Nausea. During this period, they fly a short distance to a new spot. This means that Cicadas can only be killed with ranged weaponry, as a player will be unable to hit a moving Cicada whilst blind.