Falcon-horn of Fredemund

(One use only)
This ancient horn was used by the Grail Companion, Duke Fredemund of Aquitaine. When blown, this born emits a piercing cry and the skies become filled with all manner of birds, in memory of the great man.
The Horn may he sounded at the start of any Bretonnian turn. Until the start of the next Bretonnian turn, no enemy unit may fly - they are forced to use their ground movement. Note that this may also alter the enemies' flee pursuit distance.