Noticed a rule error.
Fey Death grants an immediate reaction use of Fey Step when you reduce a target to 0 Hp. But, you can't use an Immediate action on your turn. So the only time that this could be used as an IR is if, say, ongoing damage you caused killed the target.
I assume it means that you get to deal sneak attack damage even if you don't have combat advantage, if you score a critical hit with an appropriate weapon.Fey Death can be fixed by making the use of fey step a free action.
The one that really makes me wonder what they had in mind is the Epic Tier feat Critical Ambush:
Benefit: If you score a critical hit with a weapon you can Sneak Attack with, you can apply your Sneak Attack damage if you have it available.If you have your sneak attack damage available, and you are attacking with a weapon you can sneak attack with, can't you just apply your sneak attack damage as normal?