This unicorn horn is slightly different from the normal unicorn horn that is obtained by killing unicorns at various locations around Gielinor. This horn can only be obtained by crushing a unicorn that is trapped in a cage during the Underground Pass quest and when examined it indicates that this horn is withered compared to horn of a normal unicorn.
The player must first search the cage that contains the unicorn to obtain a piece of railing and then use the railing to lever a rock down onto the cage, crushing both the cage and the unicorn in the process. The player can then search the remains to obtain this horn. Later in the quest, the horn must be thrown down a well along with three paladin's badges.