what dangers might rainsford face while going through that island ?

THe island has the ominous name of "Ship Trap Island" so that isn't a good start. Zaroff warns Rainsford of the "Death Swamp" which involves quicksand. THere are apparently wild animals, cliffs....as well.

you didnot get it ... i mean the grasses he encountered in such dark night , maybe the silt of the water while he approaches the island ...mmm