How do illusionary offensive spells work, such as a CrIm for POF?
I thought I saw rules for it some place but not sure what book/section
Are you looking for some sort of rule that covers what happens when a character is presented with an image of something that, if substantial rather than just an image, would have injured or killed them?
There are not any such rules. I don't believe that these situations should be handled any differently than any other imagonem spell.
Badly.
In previous editions you could create the illusion of pain. You can't do that anymore: well, not really effectively. So, they -look- impressive, but the trick is to aim the mock pilum so it just misses and the person fails a Brave check and runs off, preferably at full speed into a tree that you had made invisible.
ok. I could easily have been thinking of another system where the illusionary damage translated into fatigue or something, and the target eventually passed out when it would have "died"
Yes, that would be your 1E/2E/3E/3.5E showing... you'll want to tuck that back in. 
-Ben.
Or maybe it has something to do with Cause and Cure? Or am I mistaken? I seem to remember a place where Im spells were somewhat real.
Fixer, are you thinking of Glamours perhaps?
Nope, but I'm maybe confusing things, this is very foggy in my head. I seem to remember a place were imaginem could replace any other form, but can't seem to remember if it was in cause an cure or some dream place in a mystery book. And I'm too lazy to search 
Dream Magic. The Mysteries.
The Merinita Major Mystery Hermetic Virtue, Glamour. It basically allows to substitute CrIm and MuIm for any other Cr or Mu spell. Cool/flavorful and rather useful virtue to initiate, as most of the Merinita tools. 8)
I think the ghostly spells of the magi of Calebais might be what you're thinking of?
Or maybe a blend of all of the above 
(Senile mode )
it's probally a carry over from illusions in white wolf that I was thinking of
There was a phantom BoAF dealing true damage ( 
) in Wizars's Grimoire 4rd Edition but I think actual rules forbidden it
That's not surprising the actual rules of fourth edition forbid many of the spells in the 4th ed. WGRE.
Glamour magic can do this in fifth edition (as wanderer points out above). That's a heck of a powerful virtue.
Merinita Mysteries rule. Solid illusions, extra nifty durations, creative boosts to spells, summoning faeries out of anything, arcadian teleports... I love my HoH:MC 8) 