Originally Posted by
Eth
First off you should try to look at the single things deception on these items can do or not do.
Sneak attack damage
Sneak attack attack bonus
Procing bluff (turning something around and flagging it for sneak attacks)
Items:
Category 1:
The effect deception on all items prior to U19 only effects the bluff proc (Seal of House Avithul, Golden Guile, Ring of Deceit, Agony)
Note that there are items here that have deception and some that have improved deception. The first has a 5% chance to proc bluff, the latter has a 10% chance.
Damage/attack on sneak attacks was a seperate effect called Sneak Attack Bonus. This bonus could be of type enhancement or insightful.
Category 2:
After that there are items that have all the 3 things listed above combined under the term deception (I think Strange Tidings is the only named item where that is the case, the effect also existed on random lootgen).
Category 3:
Items from U31+ with deception have the effects sneak attack damage and sneak attack attack bonus combined, but no more bluff proc.
Now what stacks?
Attack and damage have versions with enhancement and insightful bonus. Those stack. Whatever categories of items above you combine only the highest enhancement and the highest insightful bonus will apply.
For the bluff proc the highest one works. So for category 1 this would be improved deception with 10%. Some people claim however that improved deception stacks with the deception from category 2 or 3. I have no data to confirm or deny this.
You can stack the bluff proc if you use one item in an equipment slot and one item in a weapon slot (for example Agony). This was tested and confirmed by nokowi.