I guess I'm not sure what you meant by invisible octogons on the land.
In my mind, these octogons aren't like mana pools that you come across every now in then. Or hexagons, or squares...probably either of the latter.
They are part of a unique magic system thats never been seen, that I haven't thought of. In this system the magic user can use all the land to affect his magic somehow, or all of the land ties into the use of his magic.
The herbs and trees wouldn't have to glow.. although I can imagine a cool cutscene in the story of this imaginary game where some high level teacher NPC allows for the player to temporarily see glowing magic in all things ala the matrix...
of course, the trick is that during actual gameplay the magic is just certain values like +3 life
or -14 earth inside those invisible hexagons laid out across the map, values that perhaps only the code inside the game computes.
