What is the scariest model in 40k tabletop lore wise?

What is the scariest model in 40k tabletop lore wise?
Chaos Demon Horros? Necron Flayed Ones? Sly Marbo?
I am geniluly wondering what could spook the spookers.

space marines

chaos spawn would be pretty demoralizing to look at

Culexus assassins.

Everything's terrified of them, even shit that doesn't feel fear.

Hell, in the older editions of 40k, they used to reduce the leadership of everything near them, ally or enemy, no matter what it was. Nids, Crons, didn't matter. Something about the Culexus just scared the shit out of everything.

[x] agree

Maybe this is normie tier shit, but if one was say a guardsmen and saw a chaos space marine or maybe an arco flagellent, that would be pretty fucked up.

Imagine you're a farmboy with a lasgun, scarcely three months of training, and no foreknowledge of Chaos beyond your Commissar's constant raving that only can save your soul from the Dark Gods'' horrific power. Now imagine you're sitting in a trench with several hundred lither recruits, when several tens of thousands of these come barreling across No Man's Land and could not care less how many times you shoot them. Imagine how long you, you personally, would hold your ground in the face of that.

Now I have to go read the Admech Trilogy again.

*only death in the Emperor's name

Assorted Tyranid biotitans - look less fucked up than Chaos stuff but makes up for that by horror of the sheer size.