Why the  sacrificial grass  (Kusah or Darbha) has got 

 such a holyness for  all the Hindu Rituals. ?

In the Mythology, to remove the slavery of his mother Vinata from Kadruva, the King of Birds, Garutman (Garuda)  went to Amara Loka and brought flagon contains Ambrosia (Amruta Kalasah) and kept on the Kusah (Sacrificial Grass) and shown to cousins ( step mother’s , Kadruva’s sons) and asked to liberate his mother’s (Vinata’s) slavery from  their mother. Because of placing the Amruta Kalasah ( pot containg Ambrosia) the Sacrificial Grass (Darba) became sacred. For this reason, since then  Kusah (Darba) has become headmost and worthy for all the rituals of Godly Sacrificial Fires (Yagnas and Homas etc.,). In the Mythology the mighty Demon Vrutrasura was killed by Indra Deva. Then that Asura fell on the gound and became as Kusah (Darbh). Hence Kushah is treated as sacred in all the  Hindu Rituals.