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Most published adventures tend to avoid anything that is seriously beyond the players.
Step 1: Tell the players flat out, at the beginning of the campaign, that there will be encounters they simply cannot handle. Some people may love that. Others may hate it. But they need to know (I've had so many problems in games I'm in due to players and the DM having different expectations and not realizing it).
Step 2: Unless you're very good at it, don't spend your time trying to describe how deadly the creature is in abstract terms. Just tell them that their characters (especially whoever made the knowledge check) don't think they can take it.
One thing I've very rarely found is a DM using a knowledge check to give a relative power level. I've seen descriptions of creatures far weaker than the party that sounded no less powerful than descriptions of creatures far more powerful than the same party (at the same point in time). Maybe every knowledge check should begin with an estimate of how dangerous the foe is, in terms of CR vs. APL (but, presumably, not the actual numbers; think an MMO's color-coding).
The Worf Effect works if you're in a position to introduce the foe early, stomp some NPC or creature, and let the party walk away to level up before coming back. It's very awkward to do repeatedly, though (how many NPCs of known capability are available? Ok, you can use other monsters, but then the players need to know how powerful the monster is relative to the party, to be able to judge whether that demonstration is meaningful. Which works best if the party has fought a lot of that monster.
Recent example from a game: a monster trivially destroyed a pair of frost worms. This was supposed to communicate to the party how much damage it could do, and at what range it noticed and attacked creatures. All we got out of it was "some sort of disintegration ray, ok". Never trust your players to understand what you're trying to communicate unless you are saying it straight out.