In this video guide I will show you how to close the rift leaking void creatures in Icebind Cavern. So in order to even get to the area where the rift is, you will need to continue with the mission until you get a new ability for Strife. You will be able to create portals to go through essentially. You will get this when you enter the dungeon. Its pretty much story related so you can't miss it.
There will be a portal before the boss room. So make sure to take this and go back to the first floor. Then head North on the map until you see where I am on the map. Or find the structure where you can use the Strife ability. From there head through the portal and continue on to the new area, Once behind the wall head to the left, hop over the gap and the rift will be right there. Activate it and kill all the guys that come out, and the achievement will pop.