Record of Agarest War Zero

Achievement - They only come out at night. This achievement leads credence to the scary monster phrase, however, no certain time of day is required to obtain this 'chiever ;). This achievement simply requires that you kill at least 1 of monsters 147-150 in your monster compendium. You will need to follow the True Path as at least one of these monsters (Jatayu-147) is only found in the latter part of the Boundary Plane(location Minor Lament).
Kill each of these 4, and the achievement will pop as soon as the last one drops. The battle does not have to be won for this achievement on the final kill, as I got my achievement to pop as soon as Jatayu dropped and I still had baddies left on the screen.

This achievement is for killing 4 rare monsters. They are numbers 146-149 in the monster book.

Japar - Fenstermacher Plain (dungeon) - Place of the Dark Dawn (last battle spot in the middle of the map)
Jatayu - Boundary Place - Minor Lament. I've also seen Jatayu in one or two of the boss battles in the Boundary Edge.
Sandworm - In the Boundary Plane, after killing most if not all of the bosses, you'll have an event and then a battle, and in that battle will be one of these. I believe you also fight it before this as a boss in the main story, but you'll see it here either way. You can capture one if you like.
Belladonna - In the Boundary Plane, after killing most if not all of the bosses, you'll have an event and then a battle, and in that battle will be one of these. It may appear elsewhere, but you'll fight one here either way. You can capture one if you like.

All of them are typically much stronger than other monsters in the area, and usually have high AVD. Unless you have DLC stuff I wouldn't suggest hunting them until you are quite strong.