Owl of the Watchers
On the Isle of the Watchers there are a total of 9 interactable Owl of the Watchers:
/way Azsuna 44.2, 72.4
/way Azsuna 40.6, 73.1
/way Azsuna 40.5, 75.1
/way Azsuna 43.2, 85.3
/way Azsuna 43.6, 87.5
/way Azsuna 46.0, 84.1
/way Azsuna 47.5, 84.7
/way Azsuna 37.1, 82.2
/way Azsuna 50.4, 91.7
Upon interacting with one of them, you will get one of the following 4 buffs/abilities:
Multiple people have further investigated this on reddit, speculating that the buffs must be picked in a certain order, alluding to the fact that it might be a puzzle designed to be completed in a sequence. However, to this day, the Owl of the Watchers remain a mystery.
The fact that there might actually be a secret behind the Owl of the Watchers is also hinted at by Maiev Shadowsong in Heroes of the Storm:
Why yes, I do know the secret behind the owl statues on the Isle of the Watchers. All wardens do. We just won't share it with you.
You can hear this voice line at 1:11 in the following video: https://www.youtube.com/watch?v=hkrdcNCgRng&t=71s