I was just thinking about this.
Adding such a line to the description is a great first step, but probably the danger evaluation (and the tension formula) should also be improved.
One thing I came up with is this still pretty raw idea, I'm not finished fully fleshing it out and thinking it through: the experience value of a monster is a fairly good metric of difficulty (or at least it should be). Much better than HD, damage, HP, or other monster statistics alone. Comparing exp value to player level is one way to measure relative strength, but player level is a really poor metric for player stength. Comparing monster exp to what your char is used to dealing with is probably better. So why not track the exp value of the last bunch of defeated monsters (something like the 10, 20, 50 last kills, possibly even using two list lengths and using both to provide a good danger assessment), and the monsters seen during the last x turns?
This would allow for calculating average exp and standard deviation and determining whether a monster is in the expected range of difficulty. Also recording the top few strongest monsters and top average exp would help in giving more detailed danger messages and deciding when a -more- prompt is appropriate.
Examples for more detailed messages (if something more detailed is actually desired):
- This is one of the strongest monster's you have ever seen. (strong uniques, highly OOD monsters)
- This monster seems (far/somewhat) stonger than the last few you fought, but you have dealt with similar danger before. (OOD, but you have dealt with similarly powerful enemies before)
- While not (much) more powerful than the last few enemies you killed this one is (far/somewhat) more dangerous than what you usually deal with. (branch ends)
Also it would allow adjusting tension better both to the current situation and to character strength, which would pave the way for more interesting god wrath and other tension related effects. For example on a branch end, tension should be continously fairly high, even if nothing immediately dangerous is in sight, because there is a high probabilty something that can kill you is lurking behind the next corner. On a regular level with maybe a single OOD monster tension should only be high if there is an immediate threat. Also, if you have successfully dealt with similar danger before the current situation is probably not as dangerous as the last time.
A mechanic that might work well for this is having some base tension (independent of monsters in LOS) if the last few enemies you have seen are more powerful than both the last few you killed and what your character is used to, and if the last 10 enemies you have killed are far more powerful than the average power of the last 50 enemies you have killed.
A problem with any system like this is that tension would depend on past player actions, so it the game would judge some situtions differently depending on playing style. This might be relevant with Xom or during god wrath, leading to gamey strategies like running away from OOD monsters when worshipping Xom so you have higher tension in regular fights. Most of these exploits would probably not be worthwhile, but it's something to keep in mind.
Things a good tension system needs in my opinion:
- Tension is represented by a single value (possibly two, one of them adjusting for low HP/MP/etc., the other one assuming full strength).
- Tension represents actual danger.
- Corolarry 1: it cant just depend on what is currently in your line of sight (if you duck behind a corner that executioner is still going to catch you)
- Corollary 2: an accurate assessment of both player and monster strength is required
- Tension should not depend on things the player can quickly and easily influence, like inventory content.
Something I'm not sure about is whether tension should be adjusted for player psychology. For example if the last few fights were easy and you encounter a strong enemy it is more likely to kill you than the same enemy in a series of hard fights (each of which is started at full HP and MP). Some ways to calculate tension based on past encounters will automatically include such an effect, this may be desired or not.