Will had a special potion on his list of candidates for Potion Transfiguration.
It was the potion Voldemort had used to protect the Horcrux in the Inferi-filled Cave.
Anyone who drank it suffered terrible mental and physical torment.
First came horrifying visions. The potion dredged up the drinker's most frightening, painful memories and subjected them to intense mental anguish.
It also caused searing pain throughout the body and an unbearable thirst for water.
Turning that potion into a spell would make it useful in battle. After all, even Grand Wizards like Dumbledore and Grindelwald had painful memories.
In a fight, a spell that bought its caster even a second could decide the outcome.
No one knew for certain who had invented the potion, but Will suspected Snape had created it, either alone or with Voldemort.
Voldemort had designed the Cave's defenses as a whole, but the potion itself had most likely been Snape's work.
For one thing, Snape was probably more skilled at Potions than Voldemort. He was widely recognized as one of the foremost masters of Potions alive.
And the potion's effect made it hard for Will not to think of Snape: it summoned the drinker's most frightening, painful memories.
That suggested its creator understood painful memories intimately, which fit Snape's character and experiences.
Will doubted Voldemort had possessed memories that profound at the time.
The timeline fit, too. Voldemort had used the House-elf Kreacher to test the potion defenses in the Cave around 1979. Snape had joined the Death Eaters after graduating from Hogwarts in 1978, giving him a full year to make the potion for Voldemort.
Will wanted to improve—or reverse—it, creating a potion that brought back a person's happiest memories and perhaps even showed visions of happiness yet to come.
That was the gift he wanted