In the story, Tom refers to finding the spell for the horcrux in "Alighieri, canto three". He is referring to the Inferno portion of the Divine Comedy by Dante Alighieri about a journey into hell.
Memory spells work differently on muggles than they do on wizards. Whereas a wizard wouldn't be able to recall something if a memory charm was used, a muggle would recall it only as a very intense dream. Think of the paintings by Dali! Dark wizards could then hide information in plain sight secure in the knowledge that few if any wizards would look in muggle art, literature, or music for magic.

Older readers may recognize the very famous last line of the spell.
Lasciate ogne speranza, voi ch'intrate
The rest of us will need to Google it. Trust me, it's worth the look.

The event to which Tom refers to, where Grindelwald claims to have seen the sun rise twice in one day, was the Little Boy atomic bomb detonation over Hiroshima on August 6th, 1945.
The symptoms that Grindelwald evidences in the beginning of he story are indeed radiation sickness, made worse by his own Horcrux.

i see that the only person with feedback is yourself, i have decided to change that... verrry interesting! i am curious about grindelwald... i know that he is the dark wizard dd defeated... your story intrigues me! keep up the good work! very clever!