Wraithform

The caster becomes insubstantial. In this form, the caster can only be hit by magical attacks, magical wapons, or creatures who can be hit only by magical weapons. Ethereal creatures can also affect the caster. No attack from the caster can affect any creature, with the exception of etheral creatures, who the caster can affect normally. While in this form, the caster can pass through small holes, narrow openings, or even mere cracks, with all of their equipment. The caster is not granted the ability to fly, however. Additionally, undead of all sorts will percieve the caster as one of them, and thus won't be immediately hostile. An intelligent undead such as a lich can save vs. spell with a -4 penalty to recognize this as a magical effect.