Not sure exactly why someone would want to do it [tap intot he plane of lost dreams], but I'm thinking it'd be for some noble reason, that just happens to destroy life as we know it.
This, I like.
* He was the counselor to a great, ring-spanning nation [or a nation on another world, perhaps]. This nation was ruled by a paladin-queen of legendary justice, vision, strength and compassion, who was gradually leading her realm to a state of harmony, uniting the entire world with her vision of the way the world could be. The paladin-queen had been a general in one of the factions of the Millenium War, a seeminly endless conflict that had enveloped the land since time immemorial. Only she could bring peace to the blood-baptised factions, using wise words when possible, and unflinching strength when not. OVer many years, she forged the ancient hatreds into a new vision of a prsoperous, unitied society, and peace reigned.
Then, she died.
And the dream of peace, held by ten million citizens, died with her.
As the world plunged into a era of carnage unparalleled before it, the counselor realised what he had to do: find the place where lost hopes and broken potentials go to die, find the Vision again, and bring it back to the world. It was the only way to honour his queen's memory.
* The villian is simply trying to find his purpose in life. Once, he was a man of consequence and power, a great adventurer who had lead mercenary armies, rescued princesses, slain dragons, reunited warring clans and won and lost more treasure before breakfast than most kings do in their lives. Every bard told his tales, every child wanted to be him, every monster feared his name.
But he grew jaded. Being reputed as able to do anything, he came to beleive he could, and those few things that proved impossible chaffed him awfully. He battled his way through Trema's underworld, slaying countless creatures and shattering chthonian empires as he plunged ever deeper... until at last he came to the palace of the daemon king at the centre of the world. After disamring the king's greatest champions, the legendary character demanded a boon from the daemon king, who naturally agreed.
"I want to do something new..." he instructed. "I want new challanges, new vistas of potential. The opportunity to do such things as I have enver done before..."
"Granted" laughed the king.
I'm sure you can guess what happened next.
All the hero's acheivements were stripped from the Coda, the chronicle on the Plane of Time that records history and damend to the plane of lost dreams. Only a fraction of his personal power remained (ie, he's now an appropriate level to challange your aprty

), and great gaps opened up in his memory. His deeds and reputation were entirely lost to the world, and when he returned, none knew him, and laughed at him for a fool when he claimed to be their champion.
Brooding bitterly over his loss, he finally settled on a plan: hsi epic deeds must be preserved somewhere on the plane of lost dreams, and he would do whatever it takes to free them... no matter the cost.
(Chades of Planescape:Torment, methinks.)