This question came up with Ra's Solar Flare card, which says "Either Ra deals himself 4 psychic damage or this card is destroyed", though Tachyon and possibly other heros have similar card effects.
First off, I'm assuming that the damage can be increased/decreased/redirected/change type and/or be blocked by damage immunity and Solar Flare won't be destroyed. Otherwise the card would be extremely fragile, although this is a bit different from how I play Bunker's Gatling Gun where I assumed he must have a card in his hand to execute the 'discard 1 card' option.
But where I paused was when the Realm of Discord card "Ethereal Bonds" said that Ra "cannot deal damage". My first instinct was that this was no different from if Ra was immune to the damage, but then I wondered if it actually prevented me from choosing to do damage.
The short version: If Ra "cannot deal damage" at the end of his turn, does he have to destroy Solar Flare?
The first part you have right. The damage can be increased, decreased, redirected, etc, etc, etc...
The second part is actually a really good question. I'm not sure on the answer here. I'm inclined to say that you'd have to trash the card. Not dealing the damage in the first place is different from dealing the damage and then having it be redirected or prevented by damage immunity.
A few other cards, like Sonic Mine, or Ensnaring Brambles would have similar effects.
It's been said here by official folks that if no damage is taken then no damage was dealt. So I would say that if the damage was redirected away from Ra, then he has not dealt damage to himself and the card must be destroyed.
I will weigh in on the opposite side, then. I can see how damage immunity is a different thing than not dealing damage, but I don't really see why that would act differently here.
I think the difference here (but I could be wrong) is the choice. You can choose to discard the card or you can choose to deal damage.
With immunity you can choose to deal the damage (which satisfies the Solar flare card) but you are immune to the type of damage it inflicts so it deals no damage to you.
With the Realm of Discord you are prohibited from selecting the damage option and must therefore select the discard the card option instead.
This seems to be an (another?) instance where the ambiguity in the verb phrase 'to deal damage' between 'initiating an attack' and 'causing damage to be taken' is causing uncertainty. I think the intention is 'initiating an attack', but that's just my guess.
The more I think about it, this definaly seems like the right ruling for "cannot deal damage" and Solar Flare. On the one hand I would allow him to do other things that normally deal damage (like playing an ongoing that has him deal damage when it enters play, or using a power that deals damage and draws a card or destroys a card) and I would just skip over the deal damage instructions.
But with Solar Flare I keep coming back to the fact that it instructs you to do one of two things, and another card says directly that one of those things cannot be done. It is a shame because no damage is an especially harsh punishment for Ra, so letting him have Solar Flare out for free seemed like a nice way to soften the blow. I guess he should just be careful to keep his HP down in the Realm of Discord.