Celebrim
Legend
Yes. Good choice. And I'm please to see they've stayed fearsome in the new edition as well.
As a 1e DM, I was always on the look out for monsters that could remain challenging no matter what the level of the party. In every edition shadows have been high on that list. If you didn't have a high level cleric to turn them, they were nasty.
What you have to be on the look out for are monsters with what I call 'level invariant' powers. These are abilities that just bypass traditional player defenses in some way. The great thing about these monsters is that they never went away. You could face them early in your career, and you still could be facing them later in your career.
In 1e, the Korred was an interesting example. Among a ton of minor attack powers, they had an attack that got stronger the more charisma the target had. An example that worked in both 1e and 3e was the Fire Mephit, which had a 'save partial' breath weapon and the ability to cast magic missile. Since most characters regardless of level could be attacked by one or the other and always take at least some damage, this meant that a small flock of fire mephits always remained at least a minor source of attrition.
One of my personal favorites minor foes in earlier editions was the Stirge. Touch attack + constitution damage. I'm not sure what it looks like in 5e, but I hope it translates.
In 3e, any swarm of fine sized creatures no matter the HD was a potential problem because they had complete weapon immunity. If you didn't have a spellcaster with an area of effect attack around swarms were always a problem, and in any edition enough swarms represent a force of nature you flee from - not a monster you try to fight.
Many of the oozes that have weapon immunity are similar. Again, showing my 1e roots, one of my favorites has always been 'yellow mold'. It's got a nasty lethal area of effect attack, and yet, at low level if you make it explicit enough its just a minor hazard. But did you know that in 1e the mold could develop psionic powers?
The only monster I've seen instill real terror was the 1e vargouille with the unhealable bite attack. They were a very weak monster, but they had the ultimate threat.
