A wide-angle disintegration beam is the most powerful attack that an angry god can make against you.
When you anger a god, he/she will sometimes strike you with a lightning bolt from the heavens (see possible negative effects of praying). This attack would normally kill an adventurer outright. However, if you have shock resistance, reflection, life saving, or are engulfed by a monster, you will survive the bolt. If this occurs, the god will follow up with a wide-angle disintegration beam.
If you are still engulfed, the disintegration beam will hit the engulfing monster instead. This occurs when the monster is shock resistant. In vanilla NetHack, the only engulfing monsters with shock resistance are energy vortices, which are also disintegration-resistant, so the disintegration beam will have no effect.
Otherwise, the beam will strike you. First, your shield, cloak, body armor and shirt will be disintegrated, unless they provide reflection or disintegration resistance, or are worn under such an item (a shirt worn under silver dragon scale mail, for example).
If you do not possess disintegration resistance the beam will then kill you outright, though you will survive if you have life saving. Note that reflection will not save you, though it might save some of your armor as described above.