Damage Immunity and Deathmark

I don't think so based on the interaction with exposes and damage immunity. But Rey is immune to percentage based health effects anyway, so that might explain what you're seeing better than damage immunity.

You could test your hypothesis using DR against Jango lead or a droideka in the leader position OR use death trooper against one of those toons.

I just fought a Rey with my DR on TW, deathmark does not reduce Max health, it does that quanitity of dmg

For example, if an enemy has 10,000 HP, hitting him will deal 5,000 HP plus the dmg from the attack itself, but his Max health will still be 10,000

And no, it does not "surpass" dmg immunity because it is not reducing max health, however, AFAIK, a character Max health CAN be reduced even if he has dmg immunity, for example GAS hitting the toon without protection

TL;DR
Deathmark does not reduce max health, it does damage based on it
Dmg does not surpass dmg immunity, but max health reduction does

Thanks for the clarification on Deathmark. What about characters that do reduce Max Health, like GAS? The last paragraph of The Chosen One (unique) says:

"If the target enemy has no protection when General Skywalker uses an ability, reduce their current Max Health by 20% for the rest of the battle (stacking, excluding raid bosses), which can’t be resisted."

Unfortunately that's a lot harder to test. I guess I'll try it in the upcoming LSTB against a Droideka.

"mariogsh;c-2155683" wrote:

Dmg does not surpass dmg immunity, but max health reduction does

"Gjarboni;c-2155891" wrote:
Thanks for the clarification on Deathmark. What about characters that do reduce Max Health, like GAS? The last paragraph of The Chosen One (unique) says:

"If the target enemy has no protection when General Skywalker uses an ability, reduce their current Max Health by 20% for the rest of the battle (stacking, excluding raid bosses), which can’t be resisted."

Unfortunately that's a lot harder to test. I guess I'll try it in the upcoming LSTB against a Droideka.

Your question was already answered. This is (relatively) easily to test against GL Rey teams. It works fine (note that this doesn't apply to her when she's in her ult stance - it doesn't work there, only when the damage immunity at 50% health kicks in from her unique).