Question about Sunder + Conviction

You have to remember that Conviction works at 1/5 effect when breaking an immunity regardless if you have a sunder charm or not. Conviction only works at full effect if the enemy was not immune to begin with.

Say an enemy has 100% Lightning Resistance (lowest possible to be immune).

• Without Crack of the Heavens, your Conviction would break the immunity. It would work at 1/5 effect while breaking the immunity. A level 33 Conviction applies -150% to enemy resistances, but at 1/5 effect that's -30%. Add the -19% from your Griffon's Eye and the enemy will end up with 51% Lightning Resistance.
• If you equip the Crack of the Heavens, the sunder effect sets the enemy to 95% Lightning Resistance. Conviction then again works at 1/5 effect (-30%) and coupled with the -19% from Griffon's Eye, the enemy would end up with 46% Lightning Resistance.

Now, take an enemy with higher resistance instead, say 115%.

• Without Crack of the Heavens, you'd still break the immunity, but since Conviction still only works at 1/5 effect while breaking an immunity, the enemy would have 66% Lightning Resistance in the end.
• With Crack of the Heavens you get the exact same result as if the enemy had 100% Lightning Resistance to begin with - they have 46% in the end.

In fact, even if the enemy has 200% Lightning Resistance (possible with some unique monster modifiers), the end result would still be that they'd have 46% Lightning Resistance in the end if you have Crack of the Heavens in your inventory.

Conversely, the maximum immunity you can break with a level 33 Conviction is 129%. That would leave them att 99% (before Griffon's Eye or other effects with -res).