How does the writer use language to create an uneasy atmosphere?

anyone even if its a vague point i don't mind i'm just really not sure what to

Well the writer says 'ugly satanic looking thing'
first he doesn't know what the 'thing' is and secondly he describes it as ugly and satanic. he/she doesn't know exactly what it is so the write compares it to 'some species of sea vulture' (simile) which has to be ugly and satanic too if he's comparing the 'thing' to it.
this creates an uneasy atmosphere because you would normally assosiate the words (adjectives?) 'ugly' and 'satanic' with negativity not positivity.

also the writer continues on to say "I could not suppress a shudder as its shadow passed over me, and I watched it's ungainly flight away towards the sea with relief". this shows that the writer is so shaken by this 'satanic thing' that he can't even shudder. through the use of imagery, the reader is able to imagine the 'shadow' of this thing and the fact that the writer feels relieved once this thing has left, also makes the reader relate to the fear that the character is feeling, so it creates an unsettling atmosphere.

hope this helps