Volitional + と without follow-up

Question

The following sentence was part of the last exercise I did:

あなたが何を見つけようと、それは途方もなく危険なもの_____。大勢の人間が私達を殺そうとしてるんだから。 A

A. に間違いない、B. に越したことはない、C. に相違ない、D. に決まっている

The right answer was D.

The thing is, I have trouble understanding the meaning of this sentence. I think I understand most of it though:

大勢の人間が私達を殺そうとしてるんだから。

That's because a lot of people are trying to kill us.

それは途方もなく危険なもの

This (is?) something incredibly dangerous

Then I'm a bit stuck. I feel like the key to understanding this sentence is that "あなたが何を見つけようと、" without any follow-up. I think I remember learning something like the "思う" or "する" that usually comes after the volitional form + と can be omitted. However, I cannot wrap my head around this part.

How should I parse this sentence? And what's the meaning of it once "に決まっている" has been inserted?

Answer 1

I feel this is a difficult question even to some native speakers... The meaning of the first sentence is "No matter what you are going to find (e.g., in this dungeon), it must be something incredibly dangerous." For this ようと, see: What are the grammar rules behind this clause, 「才能があろうがなかろうが」? and Meaning of volitional passive form

So, you are supposed to choose the correct expression that corresponds to this "must be". B ("nothing is better than a dangerous thing") is clearly wrong. A and C are close, but these are usually used in the context where you are identifying something/someone with a concrete evidence. ～に決まっている (or ～に違いない, without 間) expresses a strong suspicion, which is better in this context because the speaker knows almost nothing about what they are going to find.

太郎に間違いない。 = 太郎に相違ない。
[seeing a photograph / collating a fingerprint] It must be Taro.

太郎に違いない。 = 太郎に決まっている。
["Who ate the cake?"] It must be Taro.