Level 10 Onslaught corruption does not trigger using Toxic Rain, but gem does.

Hi there, thank you for your report.
What were your gem settings when you were using 1) Bow with onslaught corruption 2) Onslaught gem setting?

If you were using Mirage archer for the 6th link in your corrupted bow setting, that could be the reason due as projectiles/pods from mirage archer can not trigger onslaught on killing blow.

Hi thanks for getting back to us.
The onslaught support gem states that
"...grants the Onslaught buff when dealing a killing blow." meaning that it can only be triggered by killing a monster with a 'hit'.

For toxic rain, only the initial projectiles and the pod explosion are considered as a hit.

But because your main source of damage is from the DoT, where the monsters are mostly dying from, the initial projectile and the pod explosion are very unlikely to land a killing blow thus not granting you the onslaught buff.

For the [6-L Corrupted Quill Rain] testing you have reproduced to us, you have swapped out mirage archer for Void Manipulation, which adds more DoT damage decreasing your chance of killing monsters with your hits.

For the [6-L with Onslaught] testing, because your 6th link is Onslaught Support, your DoT would have been less increasing the chance for the initial projectiles and pod explosions to deal the killing blow. This could be why you were getting onslaught buff more often.
If you try killing monsters with your pod explosions(without any supports) you should be gaining onslaught buffs more easily.

Hope this cleared things of for you.