Source: Champions of Ruin
You can summon a familiar from the Plane of Shadow.
Prerequisite: Krinth, summon familiar class feature.
Benefit: Your summoned familiar looks like a dark, shadowy version of a normal familiar. It is insubstantial and gains the incorporeal subtype (see Monster Manual page 310).
A shadowform familiar is more closely bound to you than a normal familiar, and the maximum distance from you the shadowform familiar can travel is 30 feet per caster level. If you are separated from your shadowform familiar by a distance greater than this, the shadowform familiar dissolves into nothingness and is considered slain.
Special: This feat does not transform a normal familiar into a shadowform familiar.