Horatio was unconvinced that there is a ghost of Hamlet until he sees him with his own eyes. Before Horatio sees the ghost he said it was only a fantasy and will not let belief take a hold of him but when he sees the ghost, his attitude changes. He is terrified by the ghost.
When it leaves Horatio trembles and looks pale. He says he wouldn’t have believed if he had not seen it with his owns eyes. He now believes that the ghost of the n=”majesty of buried Denmark” is real.
Hope this helps.