On play/On death sounds like a good theme for
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hybrid. But that's beside the point, let's focus on Duality for now.
As for Z's questions:
Light and Darkness have in common the ability to disregard their opponent's strategy, at least thematically. Darkness steals all their opponent has and makes their plans futile (exemplified by Devourer, Vampires, Steal, nightmare), while Light simply develops its own powerful plan and has all the tools to stop his opponent from mattering too much (Hope and Miracle along with powerhouse creatures are what I'm thinking). Max HP manipulation, buffs, quanta generation and healing are also strong overlaps.
Together they boast a huge number of duos (Siphonary, VDagger, Dark Domin) which all feature the style I mentioned above of not caring what the opponent has since their base is so strong. So I'd say that the facet of Domination is what they do best together.
Light and Darkness's common strengths actually feature directly opposite mechanics with same underlying implications. See SoV against SoD, Devourer vs Solar Shield (One prevents opponent's offense for quanta, other uses offense for quanta), VDagger vs Miracle (steady cheap healing vs expensive burst healing). So Conflict seems like a proper theme.
One can also take inspiration from the art for the Duality mark, which kind of looks like a hand of darkness taking over the sigil of darkness. A card representing this Grip would be a good idea.
What about: Grip of Blood | Grip of Vice (Cost 4/3 Duality, cost is so low due to the temporary nature of the effect)
Target creature gains Vampire and switches sides for 2 turns
Has the giving nature of light if used on own creatures, and the stealing mischief of darkness when used on opponent's creatures. Might make a potent stall strategy where you steal opponent's attackers to heal up, and then send them back with antimatter. Or maybe it can be used just standalone in darkness/light monos to further help their strategy of being above the opponent's strategy.
If we don't go with the conflict/grip theme, periodic/pendulum like might also be a good theme. But that's a difficult theme since light/darkness have no cards of their own which emphasize time. To me they feel to embody the inevitable instead.
Other ideas are Light and Shadow (maybe play on Wave Interference which is characteristic of EtG's style) and Moonshine/Twilight as a misture of Light and Darkness. Phases of the moon might also be a good theme to represent their cycle. Although cycle might be a better concept for life/death or time/aether
Poll
