D&D 5E Scariest Monster Ever

What is the scariest monster/encounter you have ever experienced?

High Level Monster vs. Lower Level Party. "Run!"
Votes: 8 11.6%
Ambush! "They all get to go on the Surprise Round."
Votes: 7 10.1%
Reinforcements Arrive. "I think we can take th--Oh, crap."
Votes: 4 5.8%
Assassination Attempt. "Our cleric just collapsed in a pool of his own blood!"
Votes: 3 4.3%
Sudden Disappearance. "Hey, where did the Paladin and Fighter go all of a sudden?"
Votes: 1 1.4%
Conspicuously Weak/Innocent Monster/NPC. "Steve Buscemi in Con Air."
Votes: 4 5.8%
Unseen Somethings. "They're picking us off, one by one!"
Votes: 15 21.7%
Little Girl Wandering Alone in a Bad Place. (Enough said.)
Votes: 10 14.5%
Nasty Trap. "What? This lever?"
Votes: 0 0.0%
Something Else! Please explain below.
Votes: 17 24.6%