IMPORTANT BUG REPORT
I already made a discussion about this topic but someone told me that I posted it in the wrong section and told me to put it here, so that's what I'm posting again.
I just tried a new campaign witht the sisters of twilight in very hard difficulty on immortal empires. I finished the ritual of rebirth of the Witch's wood by turn 29. Ariel didn't spawn, however it is written it was fixed in 3.1 patch... I waited one turn to see if she would spawn after this one turn but nothing happened
I guess wood elves are pretty much doomed until the bug is finally fixed.