If, by some chance, he were to die tomorrow, what on earth would you do?
Literal
By-chance tomorrow he [subject-が] dies-[conditional-たら], you [topic-は] really what do-[question-か]?
いったい ('on earth,' 'the heck,' 'exactly') is an intensifier added to wh-questions for rhetorical emphasis or curiosity. It doesn't add information, just urgency. Paired here with the 万一 + たら conditional to make a dramatic hypothetical.