How do I introduce my BBEG’s power without my pcs rushing in to fight them?

I made a homebrew creature that’s supposed to be impossible for them to fight at the moment as that creature is the BBEG. I wanted to show the strength gap between the pcs start and how strong they will have gotten at the end, but I doubt that they will back away from fighting the creature.

How do I introduce my BBEG’s power without my pcs rushing in to fight them?