Oblivion lore is very mysterious and very hard to find. So please note this is my personal take on the small part of the story we have access to.
After the English Government attacked the Ministry of Joy, they went underground. Years and years later, the Ministry of Joy partnered with the Lord Of Darkness to bring the Ministry of Joy into Alton Towers. The Lord Of Darkness wanted to get the Forces of Oblivion to the public, and the Ministry needed to research to begin planning their magnum opus. So, the Ministry helped build Oblivion as long as they could use the data gathered from the ride to test the limits of the human body and mind.
The Lord of Darkness seems to be the main leader of the Forces of Oblivion. Not much is known about his backstory or his connection to pretty much anything, but it's confirmed he's working with the Ministry Of Joy, as on the Alton Towers website.
Oblivion doesn't feature anything about smiling, so it's theorized the Ministry of Joy may have tweaked the series of events, perhaps a more forced relationship than we first figured, but there is no evidence behind this. He explains the physical and phycological effects of Oblivion in queue line videos 1 and 2.
Not much is known about the Lord Of Light, but from the visuals in the queue line video, I theorize that the Lord Of Light and Lord Of Darkness are different personalities sharing the same body. (Perhaps Dissociative Identity Disorder? I honestly have no idea and am probably completely wrong.) The two argue about the safety of the ride in the 3rd Queue Line Video, the Lord of Light explaining how the ride cars go missing down the drop, the people never to be seen again. The Lord of Darkness doesn't deny any of this, just repeating things like; "Just relax, and enjoy the experience." The Lord Of Light is very against Oblivion, perhaps partnering with Light CORPS in the 1998 fireworks show.
There is also a chance the Lord of Light could be a scrapped character, as the 3rd Oblivion queue line video was removed after The Smiler crash in 2015 because it suggested that the ride could be unsafe. However there is no evidence other than this to suggest he is scrapped and I refuse to believe he's scrapped because he's my second favorite Alton Towers character (second only to Lord of Darkness!.)
The 1998 Fireworks Show
"Good? Evil? It's all an irrelevance. What matters now is you are staring Armageddon in the face. At first, condemning the human race was simply business, but now, it will be a pleasure." -The Lord of Darkness.
Light CORPS is introduced/appears in the 1998 fireworks show. It is unknown how she is connected to the story beyond this, but it seems she has partnered with the Lord of Light. She shows up after the LOD announces his plan to set the sky on fire and curse the Earth with eternal darkness. She explains the Crystal of Light, which has been buried for millions of years, can kill the Lord of Darkness, stop the Forces of Oblivion, and save the Earth. And so, she summons the Crystal of Light.
The Lord of Darkness mocks the crystal and says he's going to smash it into 'a million tiny crystals.' But something goes wrong, as he comes back seemingly panicking and getting defensive and saying we will pay for this, but with a loud 'screech' he is gone and seems to have been killed.
"The madness is over, the Lord of Darkness is history, condemned to Oblivion once more!" -Light CORPS.
However, as the fireworks show seems to end after a large celebration, he's back.
Surprise! Did you think in your absurd naivety the Forces of Oblivion could be so easily destroyed? You may have won the battle, but not the war. In the meantime, if you need reminding of my powers, drop down and see me some time. And remember, DON'T LOOK DOWN!
So yes, the Crystal of Light may have stopped his current plans, but the LOD is still very much alive, though it's uncertain if he's received any injury or such from this.