from Latin "ultimo" meaning last
After betraying the Heren Istarion (Order of Wizards), Saruman issued an ultimatum to Gandalf -- join the Dark Lord or be beaten up and trapped in the tower of Isengard.
During the Last Debate, the leaders of Rohan and Gondor agreed to deliver an ultimatum ordering 7000 men to march on the Black Gate.
condition, demand, requirement, last chance
offer, attempt, beginning
"Here at the end of all things."
LOTR The Return of the King - The Crack of Doom