How does proc on spell work?

[fateriddle]

There are many proc on spell items, my question is when does the effect trigger for:
1. a spell hitting an area of targets,
2. a spell hitting multiple targets(earthen missile, rethread, chain lightning ... )
3. a spell last several turns.

[darkgod]

Easy: On spell *cast*, always.

[Radon26]

You are a horrible horrible person.
That is not an answer.

It would mean, that if i targeted a tile 10 away, and the projectile stopped 2 tiles away, the proc would still try to go 10 tiles away, as it doesn't CARE about the connection itself.

More confirmation is required!

[HousePet]

A better answer than the one from the Tenebrous Overlord is that it requires a spell to do direct damage.
So spells doing damage over an area could hit proc on any target hit. Damage from a timed effect does not proc.
So Burning won't work, but Aether Breach will.

When multiple targets are hit in one turn, starting with the first creature hit, each proc chance is rolled and then it moves on to each new damage hit applied and rerolls proc chances until one of them works. Then it stops rolling them until next turn.