Detect Illusion

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Is this even a thing in this game or is it a junk stat that I won't waste on my theif?

 

It can dispel "mirror image" and improved invisibility. It is rare but a few enemies will cast it.
It just isn't worth the points until everything else is at 100% because mages casting those spells are rare.

 

A 100% DI skill is like having an unlimited supply of True Sight. It's quite a useful skill in the BG games (especially BG2), where there are numerous enemies using illusions/stealth/etc which DI dispels, but less so in IWD.

 

Oh, it is VERY useful in BG2, I'm just saying that it is less so in IWD where only a handful of enemies will use illusions against you. Even at that, half the time when you encounter them you will probably be too low level for DI to be that useful unless you put ALL your points into it, such as that assassin on the steps of the FAI who will probably be your first "boss fight" that casts Horror and then Mirror Image. DI would be useful to dispell it, but not as a fresh level 2 party (you get enough to level up in Candlekeep) unless you put all your points into it.
Silkie is another early example, you probably meet her shortly after arriving in Begost which you will probably head for right after the FAI since the game points you there.

For this reason I recommend waiting until you import into BG2 to start putting points into it for the Bhaalspawn, but when you do decide to invest, go the whole hog until it's over at least 50%, preferably 75.

EDIT: I misread and thought for some reason this was the BG1 forum.
IWD1 is even worse, as there are even fewer enemies that use mirror image or invisibility against you and I don't recall any that were particularly annoying to deal with, unlike in BG1.