I tested this with the help of Dreamthorn (2h sword with total of +47 melee void damage) with and Decaying Skull (helmet that gives +80% inc void dmg).
I also had the ‘Focused Strikes’ specialization (strike 1 and 2 cannot crit, strike 3 always crits) so it would be easier to follow the damage.
The damage test were done against the dummy in Ruined Era Council Chambers.
Rive strike 3 crit dmg tests done (with Dreamthorn):
#1: Without ‘Gaze into to the Abyss’, without Decaying Skull
#2: Without ‘Gaze into the Abyss’, with Decaying Skull (80% inc void dmg)
#3: With ‘Gaze into the Abyss’ (100% inc void dmg), without Decaying skull
#4: With ‘Gaze into the Abyss’ (100% inc void dmg), with Decaying Skull (80% inc void dmg)
Expected results: Test #3 would do more dmg than test #2 (and test #1 the least and test#4 the most)
Observed results: Test #3 did less dmg than test #2
Results/crit dmg ranges from Strike 3 crit dmg tests*:
#2: 509-694 (Decaying Skull 80% inc void)
#3: 362-517 (Gaze into the Abyss 100% inc void)
#4: 532-730 (Gaze into the Abyss 100% inc void and Decaying Skull 80% inc void)
*the sample sizes for tests were small
Character stats without Decaying Skull helmet
(10 str, 40% inc melee dmg, 40% inc void dmg, crit multi 223%)