Joaquin Phoenix won his first Oscar on Sunday for his terrifying performance as an isolated loner who becomes one of the world's best known comic book villains in "Joker," and invoked his late brother River Phoenix in one of the most emotional acceptance speeches of the night.

Phoenix, 45, won the best actor Oscar after three previous nominations, crowning an awards season that has seen him sweep every major prize for his role in the standalone origin story of Batman's archenemy.

"I've been a scoundrel in my life, I've been selfish, I've been cruel at times, I've been hard to work with. I'm grateful so many of you in this room have given me a second chance," Phoenix said in accepting his award.

"When he was 17, my brother wrote this lyric, he said: Run to the rescue with love and peace will follow," he said in concluding his speech tearfully to a standing ovation.

River Phoenix died of a drug overdose at a Hollywood night club in 1993 at age 23.

The actor, known for playing brooding or emotionally troubled characters, dropped more than 50 pounds (22 kg) to play Arthur Fleck, an emaciated mentally ill clown who finds fame through a random act of violence in 1980s era New York City.

His Oscar win made Phoenix the second person to get an Academy Award for playing the Joker character. Heath Ledger won a posthumous best supporting actor Oscar in 2009 for playing the Joker in "The Dark Knight."

Dark and unsettling, Phoenix's Joker is far removed from the comic book characters traditionally seen on screen. Matthew Belloni, editorial director of the Hollywood Reporter, described it last year as "among the most chilling characters I have ever seen in film."

Publicity averse and intense, Phoenix has a reputation for completely inhabiting characters that have ranged from country singer Johnny Cash in "Walk the Line," to Jesus Christ in "Mary Magdalene" and an impressionable drifter who enters a cult in "The Master."

In 2010, he almost succeeded in fooling the world that he had given up acting to try to become a rapper in the fake documentary "I'm Still Here."

A strict vegan and advocate for the environment, Phoenix was born to missionary parents who traveled through Central and South America before settling in Los Angeles, where he became a child actor.