Looking for a toxic substance that cannot exist at room temperature I am writing a short story, and am trying to find a gas with the following properties:


It must have a very distinctive odor
It must be impossible for it to exist at room temperature under normal conditions
It must be toxic to humans


How large a pool of possible substances are there that meet these requirements? Is there a particular family or class of gases where I should be focusing my attention?

Some additional clarification (apologies for not including this when I first wrote the question):

The scenario I am writing is Cthulhu-based. The idea is that this substance is discovered where it shouldn't be able to exist. This is where the second criteria comes from. They will collect a sample of this stuff, analyse it and realise that its mere existence is impossible in the circumstances they found it. A completely hypothetical example might be a substance that spontaneously combusts in earth's atmosphere, or that immediately reacts in some other way with a gas present in air. This would mean that it cannot exist where the protagonists discover it, yet it does.

The toxicity is just intended to add some jeopardy to the situation
As Ivan Neretin already explained in his comment, you can't have it all.

Anyway, cyanogen, $\ce{N#C-C#N}$ would be a nice candidate. It is colourless, but highly toxic (H330). If it's cold enough, that is below -21 °C, you can even have a pond of it ;-)

In addition, it adds a historical space touch to the story. Back in 1910, when the return of Halley's comet was expected, cyanogen was spectroscopically identified in the comet's tail and the New York Times titled COMET'S POISONEOUS TAIL ;-).