Then Amnon said to Tamar, "Bring the food into the chamber, so that I may eat from your hand." So Tamar took the cakes she had made, and brought them into the chamber to Amnon her brother. But when she brought them near him to eat, he took hold of her, and said to her, "Come, lie with me, my sister." She answered him," 'No, my brother, do not force me; for such a thing is not done in Israel; do not do anything so vile! As for me, where could I carry my shame? And as for you, you would be as one of the scoundrels in Israel. Now therefore, I beg you, speak to the king; for he will not withhold me from you." But he would not listen to her; and being stronger than she, he forced her and lay with her. (2 Samuel 13:10-14, NRSV)
And Amnon said unto Tamar, Bring the meat into the chamber, that I may eat of thine hand. And Tamar took the cakes which she had made, and brought them into the chamber to Amnon her brother. And when she had brought them unto him to eat, he took hold of her, and said unto her, Come lie with me, my sister. And she answered him, Nay, my brother, do not force me; for no such thing ought to be done in Israel: do not thou this folly. And I, whither shall I cause my shame to go? and as for thee, thou shalt be as one of the fools in Israel. Now therefore, I pray thee, speak unto the king; for he will not withhold me from thee. Howbeit he would not hearken unto her voice: but, being stronger than she, forced her, and lay with her. (2 Samuel 13:10-14, KJV)
This is the crime that will ultimately send David's kingdom spiraling off into chaos, when Absalom rebels later on, which is undoubtedly the most terrible moment in the whole book of 2 Samuel.