Loot River

This achievement takes place in the 2nd area "The First City" and requires at least two runs to get it.

First you must find Rupert who for me was on a thin path above the merchant, probably about half way through the area. You need to talk to him, hit the wall on his left and then collect the loot behind this illusionary wall. He will then return to Sanctuary and offer spells for purchase with knowledge.

Then on a separate run, if you return to this same area, thin 1 tile path directly upwards somewhere, where Rupert was previously instead there will be Callum. Simply accept his deal and you will get this achievement.