Grand Skyfall
Active: Pantheon begins channelling. If Pantheon goes uninterrupted for 2 seconds he will leap into the air, becoming untargetable. Pantheon will then begin plummeting toward the ground at the target location within 5500 range, striking after 1.5 seconds and becoming targetable once more.
Not sure if they simply programmed untargetable status as invulnerable, if so that prob explains it.
Still I wouldn't called it a "reliable" escape considering the wind-up time, the risk of being interrupted and its epic cd.