'season's grace' - sibilance sounds like a snake. plays on the readers aural senses
'great black snake' - 'great' implies admiration and respect
'froze half through a pace'- cold imagery contrasted to the previous 'sun warmed'
'He quested' - mission orientated makes the snake appear ominous
'parted grass' - the grass moves aside for the snake, it is significant and important, treated like royalty
'We lost breathe' - role reversal of the…