Uncontrollable Phasing (3.5e Flaw)
From D&D Wiki
You will, from time to time, randomly disappear from existence for a short period of time. You then, after a certain amount of time, phase back into existence with no memory of anything that had happened in the time of your disappearance.
Prerequisite: You must exist as a physical form.
Effect: While phased from existence, you can take no damage or be attacked, as you no longer exist, but you become Shaken for as many rounds as you became phased for from your experience being literally non-existent. This number of rounds is determined by the DM and can happen anytime at the DM's discretion.
Roleplaying Ideas: Does becoming unreal change your character in anyway? Do they become enlightened from what they experienced? Do they become unnerved? Terrified? Angry? Did they see/hear/experience something while out of existence? Did they learn some form of knowledge from the encounter? Some Truth?