[Kromm]

Many such creatures fall into the category of those that would not be vulnerable to sneak attacks/backstabbing in The Other Game. Which is to say, they're intended to render Stealth useless. Bruno offered a pretty good breakdown.

Most enhanced senses don't obviate Stealth – they just reduce the number of things that give you bonuses. For instance, you can use Stealth vs. 360° Vision, but being behind your target won't give you a bonus or eliminate an "in plain sight" penalty, or force your target to rely on Hearing instead of Vision. You can use Stealth vs. Infravision or Dark Vision, but low light won't penalize your victim or give you a bonus. You can use it against Subsonic Hearing, but now it's Hush and Mage-Stealth that won't help you. You can even use it against de facto vision replacements such as Scanning Sense (e.g., Sonar) and Vibration Sense, but you'll most likely be denied the benefits of being behind your target, sneaking up in low light, and using spells that eliminate noise but not vibration.

The simplest way to deal with all that is to have Stealth high enough that you don't need darkness or magic or a turned head to use it. Take your current level and add 10. That should be good enough. It will offset the "basic -5" for backstabbing and the extra -5 anywhere but in bush or shadowy tunnels – which is to say, the -10 for trying to use Stealth in plain sight.

The brutal sense is Detect – and as the developer, I can confirm that Detect is just code for "This monster kills thieves." Don't waste time trying to sneak up on it; it's a monster you have to confront toe to toe. This includes Demons of Old, electric jellies, ice wyrms, jellies, slimes, and undead slimes. In all cases, there are ways to eliminate the thing being detected, but that relies on having very specific magic that renders you cold, inorganic, unliving, or whatever, most probably through "Body of" spells.

This is why Nondetection is such a cool ability, by the way. It lets thieves work around the major limit (monsters with weird senses that obviate Stealth . . . oh, and magic) on their primary role (sneaking and backstabbing). It is for thieves much as Mana Enhancer is for wizards, and has a comparable cost when maxed out.