This definitely seems to be a question of, "How shall I foreshadow this threat?", as well as how to guide your players away from certain doom, lest their hasty actions wipe them.

There is a whole slew of literary devices that can be used to better introduce an important conflict in your campaign. And as DnD is an interactive story, I'm inclined to agree with most here: watching a cut scene is just boring. So is fighting a monster they obviously can't defeat. Taking away a player's agency (which includes the limited options of run or die) is, in my opinion, is a terrible thing to impose upon the PCs.

But there is some great advice that others have already mentioned here, and provide favorable solutions.