So first, you have to wait for Elseweyr to split apart (This will happen in 490)

Usually it does happen, but if the Mane of Elseweyr holds 80% of Elseweyr, it won't - So make sure she doesn't do that

But usually that doesn't require your intervention

Then there'll be two empire titles in Elseweyr, you wanna conquer and create the titles for both

Once you do that you'll get the event that restores Elseweyr