In the archives are 2 or 3 versions of the same question - how many "kills" does it take to get the legendary swordsman event so that you can build No-Dachi. Well, I have had the answer for the longest time, and never realized it. Its in the Strat guide for Shogun:TW - and it states that a legendary swordsman event is triggered when a samurai (not an ashi) reaches Honor 2. Note - this is regarding any samurai in any infantry (does not appear to account for cav) unit that reaches a PERSONAL honor of rank 2. So there ya have it.
Hope this helps those who may have still been wondering.
Qapla!