Beast Quest
24 Achievements
1,000
10-12h
Hero of Avantia
You completed all of the side-quests!
100
3.49%
How to unlock the Hero of Avantia achievement in Beast Quest - Definitive Guide
More information will follow.
I have now finished the main story a second time, and the percentage has not changed, meaning it either will not work or is one of the side quests.
(edit) As of, at latest, January 29, 3:50 2020 there has been an update which solved this problem. I simply loaded up my game which had all of the side quests completed and the achievement popped within 10 seconds.
Side-Quests can be found primarily in the game's towns, aside from the ones related to the "Artful Adventure" achievement (see that for more info). You will see a gold pulse on your mini-map when a quest is available and a gold triangle above the quest-giver's head (these turn green for completed quests). Most of the quests are very straightforward fetch quests or getting drops from enemies, though enemy drops tend to have a bad drop rate and could take a while. As mentioned in the Roadmap, no quests are missble so work on them as you like during the story, but don't go too out of your way, then simply finish them up after the story is complete.
Note: hit to be shown a gold glitter trail to follow (similar to Fable) to easily complete these quests. However, the trail is kind of dumb and will sometimes not show up because you're not in the right zone, so try to take note of the instructions. It will also sometimes dead-end into a wall, and I found fast traveling to the next campfire and back-tracking sometimes fixed this. Either way, it can be frustrating to complete these due to bad game design.