This feat is granted free to Dark Hunter at class level 1,5,10,15,20
This feat is granted free to Rogue at class level 1,3,5,7,9,11,13,15,17,19
Targets immune to critical hits (such as an undead, slime, construct, elemental, or anything with high Fortification) are also immune to Sneak Attack bonus damage. Effects that decrease or bypass fortification can partially break this immunity. Some creatures appear to be completely immune to sneak attacks, and won't receive them even if a large portion of their fortification is being bypassed.