It was revealed in SoaDQ that Diigai knew of the LifeStone yet when it was threatened he decided this was a great time to attack the kingdom, weakening their defence of it, but why ?

From: Raymond E. Feist
Date: 18 August 1997

There are a couple of ways to look at this. 1) the Kingdom will take care of the Lifestone issue, and we'll have gains because they're distracted, and if they don't, we won't care anyway 2) he knew of it, and didn't believe the story 3) he's just a very dumb emperor. He wouldn't be history's first, you know.

Best, R.E.F.

FAQ answers attributed to Raymond E. Feist are copyright by Raymond E. Feist.
It should also be born in mind that the answer given was only applicable on the date written, and to a specific question. You may find further, similar questions, in the FAQ.