As Ali may remember, at London RGRN we discussed a
change to unicorn mechanics to overcome the repeated
undead turning approach that generates a potentially
infinite supply of unicorn horns.
Briefly, the idea was as follows; when you cancel a
unicorn, you get a message saying that its horn
disappears. A cancelled unicorn can't cure status
ailments or grant Luck for gems - but it's still
beloved of the gods (so works the same for killing and
sacrificing) and still identifies gems via the
grateful/gracious messages. Obviously, also, a
cancelled unicorn does not leave a horn when killed.
More significantly, if a unicorn corpse is resurrected,
it comes back cancelled (unless there's a horn on the
target square to be consumed in the process?)
Hence each unicorn is only every good for one horn.