Upon donning his armor and summoning his chariot, Ravana resolves to either be victorious by slaying Lord Rama or die doing so.  As part of his resolution, he states that either his wife, Mandodari, or Devi Sita, Lord Rama's wife, will become a widow.

When Ravana makes his resolution, he had received painful news from the war front.  All of his commanders had died at the hands of Lord Rama. According to the R.K. Narayan translation, Ravana heard "the cries and the wailings of the widows of warriors."  Ravana understood that he must go to the battle front and face Lord Rama himself, saying "the time has come for me to act by myself again."  

Ravana bathes, offers prayers to Lord Shiva, dons his protective armor, and summons his chariot.  He firmly makes his resolution, to be heard by human and heaven alike:

This is my resolve:  Either that woman, Sita, or my wife Mandodari, will soon have cause to cry and roll in the dust in grief. Surely, before this day is done, one of them will be a widow.

Ravana's resolution is extremely important.  It shows the sheer force of Ravana's conviction.  Negotiation is impossible.  Victory over Lord Rama is the only metric he uses for his success. It marks the final chapter for Ravana, an end that Lord Rama will deliver.