I started a new campaign last night, running the Yawning Portal version of Sunless Citadel in Eberron for a group consisting of a mix of lapsed and current players. The first thing that happened was a tense, close-to-TPK fight against 3 giant rats. It was scary, but fun scary. But having run quite a bit of 5E in the past, I know that particular feel goes away pretty quickly. It is pretty difficult to kill 5E with even a couple levels under their belt but I think the game benefits from a healthy dose of mortal fear.
What methods have you found to keep the game dangerous even as PCs rise in level? What methods have you tried that did not work out or otherwise ended up not using? What's your take on high lethality D&D as it relates to player engagement and enjoyment?
Thanks!
Don't mistake your view of the fight for their view of the fight.
1. Bring back level draining.
2. Players will then be fearful of having their levels drained, PLUS you can keep knocking them back to 1 and they can be afraid of that, too.
3. ????
4. Profit!
Monster attack damage does not match PC HP inflation. Add that to the additional methods PCs have to recover /avoid damage as they get higher level and combats become increasing less dangerous. The two easiest options are:
1) Make your monsters hit harder, and/or...
2) Reduce the PCs HP
It is hard to convince players they should have less HP, so more powerful monsters is usually easier. You don't have to do this for every battle or every monster (you can also use more monsters and environment, hazards, traps to increase the threat).
You can also look at doing an E6 or E10 version to limit power levels as of PCs as you get to higher level. I think I am going to do this (E10) for my next campaign and see how it goes.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.