"Well... Here I am."
When the assassin heard these words, he could not help but gulp in fear. The implication behind this unseen man's speed was simply unfathomable.
To be able to arrive at his blind spot without him getting a sniff of when and how that happened was absolutely ridiculous! This assassin should know because he was a master in the arts of hiding.
And a pretty accomplished one at that!
This kind of thing just doesn't happen unless there was a huge gap in strength or this unknown master has used some kind of advanced concealing technique beyond that of his understanding.
'Fight or flee!' the assassin thought briefly. There was really no reward to be had for any heroics or useless shenanigans he might do if that would culminate in him losing his life for it.