When you refuse his "shady" offer, he never again offers it actually. So you are locked out of tenebrium weapons for 3 of 4 characters, as only -1- skill book with this skill exists, and it's in Attenboroughs office (in the hidden imacculate village, she's the geomancer there). I assume this is the "fall back" Larian added to make sure SOMEONE could handle this in your group.
