One thing that has me puzzling was, how was Oleander living during the time between becoming a dark unicorn and the events of the game? She spent 13 years training with Fred in order to beat the predators, so much is clear. But what about her home in the Woodlands? In the lore entry her punishment at the time was hilariously low (go to your room and think about what you've done.) but I cannot fathom that afterwards the high priestess would have tolerated her keeping the Unicornomicon.
This would normally leave two options. The other unicorns were unaware of the unicornomicon and Oleander managed to hide it for many years or Oleander lived in exile, either as a more severe punishment or self imposed.
One other possibility though is, that the Order of the Horn is so incredibly set in their "let the Light mend things" mentality that they left Oleander be, albeit as an outcast, thinking that she will eventually see the error of her ways and return to the light.
This would normally leave two options. The other unicorns were unaware of the unicornomicon and Oleander managed to hide it for many years or Oleander lived in exile, either as a more severe punishment or self imposed.
One other possibility though is, that the Order of the Horn is so incredibly set in their "let the Light mend things" mentality that they left Oleander be, albeit as an outcast, thinking that she will eventually see the error of her ways and return to the light.