Master Infernal Scout
Recover 25 expiator parts.
Check the minimap to see how many expiator parts have already been found.
This achievement can be difficult to complete, since new hell worlds are quickly explored. If you're in a hurry, and you have the funds, or if you just can't stand the nasty behavior that often happens on hell worlds, then your best bet is to just buy some private hell worlds. Each world (even mini ones) will give you 8 instances of recovering parts.