Ace Died? What? How? Does LUFFY Get Revenge. So sad
I won't tell you how Ace dies, but I will set you up for the arc where he does, so you can enjoy the awesomeness of the arc.
As we know, Ace was chasing Marshal D. Teach, Blackbeard. Ace catches up to him, and they fight. He is defeated by Blackbeard and given over to the Marines. With this deal, Blackbeard becomes one of the 7 Warlords to replace Crocodile.
Now, Ace is in the hands of the World Government and the Marines. The Marines decide to execute him for his ties to a certain infamous pirate. However, being a part of the Whitebeard Pirates, Whitebeard, the world's strongest man and one of the 4 Emperors of the New World, isn't just going to sit idly by and let Ace die. He gathers all thirteen of his divisions and his New World allies. They set off to Marineford, the Marines Headquarters, where Ace is going to be executed in order to save him. In anticipation for Whitebeard's actions. The Marines gather all of their captain rank or higher marines to Marineford along with the Seven Warlords to fight this force. A huge battle is about to ensue.
Luffy, after rushing to Impel Down--the prison where Ace had been held before moving to Marineford--, heads to Marineford to stop the execution with the help of some familiar faces and new faces who are all ex-prisoners.
The race is on to save Ace. The Marineford War arc starts on episode 459 and lasts until 589. The manga chapter is 442.
Law is one of the 11 Supernovas, rookies who reach the half way point of the Grand Line--Saboady Archipelago-- with a bounty of 100 million beris or more. After the timeskip he becomes one of the 7 Warlords and plays an important role in the current and probably subsequent arcs of the Manga.