The shadow appearing suddenly in the dark frightened her badly.

frightened 是动词 frighten 的过去式，frighten 由名词 fright（惊吓、恐惧）加上动词后缀 -en 构成，意思是"使害怕、惊吓"。frightened 在这里表示"使...受到惊吓"的动作已经发生。