Alchemy.Events.on_user_ban

You're seeing just the macro on_user_ban, go back to Alchemy.Events module for more information.
Link to this macro

on_user_ban(func)

(macro)

Registers a handle triggering whenever a user gets banned from a guild.

args : Alchemy.User.t, snowflake The user, as well as the id of the guild they were banned from get passed to the hook.

Example

Events.on_user_ban(:cancel_ban)
def cancel_ban(user, guild) do
  Client.unban_member(guild, user.id)
end