POINT OF NO RETURN
There is point in this game that once reached, you will be unable to return to any area of the map and collect any collectibles you have missed .
The Point of No Return is after teleporting from the Northern Lighthouse in Thorniara to Xardas Tower (Zoldren) Please check the map HERE. It is where number 29 Beliar Artefact is. Credit goes to HaKaShi at Jowood forums for the map.
You need to make
SAVE in Thorniara before reaching this point. You can then easily reload the save after the final boss and do the "Richer Than Diego" achievement for example if you wish to.
From Thorniara you can reach almost every area of the map up to Cleaved Maiden by going through the cave in Thorniara (marked in the map below) which leads to Valley Of Blood. Press A to clear the rubble. Granted there is a lot of running but it is possible to reach every area EXCEPT read below:
EXCEPT there is section of tunnels in the mountains (after the Jungle) where you fight Ahn'Bael guarding the entrance to the Divine Forge, where a gate will close. After this boss fight make sure you collect every collectible on the way, because as soon as you jump down into Thorniara, there's no getting back.