Phantoms
/phantoms is a command on Legendary Hardcore, added by one of the server's custom plugins, which stops phantoms from spawning around the player who runs it. It is not available by default: a player must first earn the Two Birds, One Arrow advancement to unlock it.
Unlocking
Access to the command is granted by completing the vanilla advancement Two Birds, One Arrow, which requires killing two phantoms with a single piercing-enchanted crossbow bolt. Until the advancement is earned, the command cannot be used.
Because the advancement requires phantoms to be present in the first place, unlocking the command means surviving the very mobs it disables. In practice this involves going several in-game days without sleeping in order to make phantoms spawn, then killing two of them in one shot with a crossbow enchanted with Piercing.
Usage
Running /phantoms disables phantom spawning around the player at night. Phantoms may still be encountered near other players who have not disabled them.