King Pelles of Corbenic (father of Elaine of Corbenic) was a friend of Morgana. But his small land was decaying and losing its influence. He tried to get a mighty ally by marrying his daughter Elaine to Lancelot, but it failed. 

The Corbenic was decaing. It was necessary to usurp more wealth and power to it from Logres – the kingdom of Supreme King Arthur. 

It was Elaine who came up with the idea. It was necessary to weaken Arthur’s kingdom. So lets send his knights to the chase for a holy relict. And let’s use her clueless son, Galahad, for it. 

Since his childhood, Elaine was telling her son, Galahad, about the Holy Grail – the bowl containing the blood of Jesus Christ. Only the greatest knight ever will ever be able to find it. 

When Galahad was at the court of King Arthur to be knighted, and there was a feast to his honer, the Charade of Holy Grail was set afoot. 

There were several steps needed.

1. Kay (step-brother of King Arthur, and his majordomos) added mild hallucinogens to the drinks. He joined because you persuaded him Arthur will leave for the quest as well (which didn’t happen) and Kay will rule in his absence. 
2. Nineve (the fairy) was doing her magic with lights.
3. Aoife (Morgana’s servant) had been dressed as a beautiful maiden in white silk to walk through the Dining Hall with a golden cup in her hands. 
4. Kay‘s hidden musicians had been playing and orchestral music. They all had been killed right after the Charade.

It all played beautifully. When Aoife entered the dining hall and hidden Nineve made the light to shine on her face, everybody grew silent.  The choral had started as an angel’s choir and Aoife walked through the hypnotized hall until, thanks to Nineve, she disappeared in the ray of light.

The rest is written in the Quest of Holy Grail.