I'm in the process of running my friends' PCs thru an encounter that truly feels deadly in the sense of "threatens the entire party, not just one character, with death/unconsciousness as a potential outcome, and puts pressure on all characters to use resources and implement thoughtful and/or creative strategies/tactics." It's an incredibly tense and poignant encounter on a lot of levels.
Six 2nd level PCs at full resources defending a brewhouse connected to a defensible crenallated tower, also with special powers "unlockable" in the scene reflecting their impending 3rd level advancement (a sort of story-driven advancement, if you will).
However, these perks are countered by the fact that the party was split in half, with half running to catch up to the mounted bad guys, leaving two with 1 or 2 levels of exhaustion. Also, one of the PCs at the brewhouse/tower was poisoned for much of the encounter.
Fighting 17 tribal warriors (CR 1/8) (though 9 of these were tweaked to be horsemen), 2 mastiffs (CR 1/8), and a veteran (CR 3).
Actual XP = 1,175
Adjusted XP = 3,525
The adjusted XP is very close to 3 times the deadly threshold (1,200 XP) calculated from the DMG guidelines.
I feel like I genuinely don't know how the encounter will resolve. There is a sense among the players that they are indeed fighting for their lives, having to adjust tactics, fall back, help each other when they're in a spot. It feels deadly.