Monsieur Chouchani was a mysterious figure who travelled around post-World War II and taught several great Jewish thinkers, including Emmanuel Levinas z"l and [yibadel l'chayim] Elie Wiesel.
Wiesel relates how a friend who had studied with him under Chouchani in France warned Wiesel that their teacher wanted to weaken the foundations of their belief and admitted that he found Chouchani frightening. That person, notes Wiesel, later traveled to Brooklyn and became the head of a yeshiva and one of the greatest rabbinical authorities of his generation.
I assume that rosh yeshiva is no longer living(?), so for history's sake, I'll ask: which rosh yeshiva was it?