Owl of the Watchers
On the Isle of the Watchers in Aszuna there are a total of 9 owl statues, 3 of which can be activated at any given moment.
/way #630 44.2 72.4
/way #630 40.6 73.1
/way #630 40.5 75.2
/way #630 37.1 82.2
/way #630 43.2 85.3
/way #630 43.6 87.5
/way #630 46.0 84.1
/way #630 47.5 84.7
/way #630 50.4 91.7
Upon interacting with one of them, you will gain one of the following abilities or effects:
The mystery regarding the statues has now been solved. Owl of the Watchers is one of the puzzles required for obtaining Incognitro, the Indecipherable Felcycle secret mount!
Archive
Multiple people have further investigated this on reddit, speculating that the buffs are required to unlock something, alluding to the fact that it might be a puzzle designed to be completed in a sequence.
Maiev Shadowsong Voice Line
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.
Dungeon Puzzle Relations Chart
Google Documents
Seje and Tellz have created google documents further investigating this secret.