It hits 5 times over 6 seconds. Everything within 10 metres is hit: creatures, doors and placeables. You are never caught by it.
Total damage: 1d8 per caster level up to 15, then +1d8 per 3 caster levels above 15. Each of the 5 hits rolls 1/5 of those dice: 4d8 per hit and 20d8 in all at level 40.
A Reflex save halves each hit. Doors and placeables take double damage.