The one that triggers Dragons as random spawns I guess. My most recent main quest is the Bleakfalls Barrow quest that I have not completed and Dragons don't spawn randomly... or anywhere for that matter. You can pretty much walk up to Word Walls in "Dragon Lairs" and it's fairly undefended...
I do not know the answer to this question, will also wait for a response

I didn't even know there's a Main Quest that triggers
Random Dragon Encounters. Would love to know this also.
Are all
Word Walls guarded by a Dragon? I'm actually only 20 hours into the game and just finished
The Way of the Voice, lol. Would be great to know if I should go for
Slow Time &
Fire Breath Word Walls now if there's not gonna be any elite enemies guarding them.