Is the devil really a liar???!!?

i believe in God, and i have read the bible , the new testament makes plenty of reverences of the devil or satan being "the prince of lies" and the o.t points out that it was lucifer was who thrown down from the heavens but he is not referred to as the devil or satan , so i think they are two separate beings, but there is nothing that actually points out that the devil is a liar, what did he lie about? because everything in which he stated was true from the beginning, sorry if im playing "devil advocate" but i just dont see it. please help me figure this out....thank you