Question: If you DON'T have a light mark, but you play this card anyway, will all creatures immediately die upon using this card? Or will it do its damage (alongside the rest of the creatures) before having each creature lose its HP?
If it's the former, you've pretty much limited this card to a small set of decks, but if it's the latter, it's still too situational and acts as a detrimental Spark/Ball Lightning that takes everything on your field out for nothing (assuming it also affects immortalized/burrowed creatures in addition).
...of course, I realize that while typing the above posts, that possibly 
might try to find a way to use this card alongside Skeletons, Luciferin, Hope, and other cards...
It would kill everything at the end of the turn. Why would you play it if you didn't have a Light mark?
The problem is then that this card becomes
exclusively used in decks that have a
mark (or unless you happened to have a 'mark of Light card' that you won from tournaments in your deck). You're right though - why
would I play this knowing that it's going to kill everything on the field that belongs to me? In duo-light decks that need to splash another element (using a different mark), this card is also out of the question again, as it would just kill everything you have.
Chaos Seed may not be useful for many decks that rely on more powerful/controlled forms of CC (Congeal, Shockwave, Lightning, etc.), but the thing is Chaos Seed can still be placed in
any deck that the player wants to include in it because of its cheap cost, surprise factor, and relative general reliability to cause chaos to a random creature (assuming you had 1
quanta). With this guy (if he ever gets implemented in-game), it would be a game over if the player didn't have a
mark but wanted to use this guy, or if this was spawned from a Fate Egg accidentally, or if it was gained by a mutation, etc.
The 'requirement' of the mark is what makes this card appear unusuable, and prevents creativity in deckbuilding in my opinion.this card would been fun if you would make it cost like 3 fire or 4 fire and you need to have a light mark since this would give the game another synergy
A false sense of synergy, since you're basically forcing this card to be exclusively used in a duo-fire/light deck, or some trio/rainbow combination of with those elements included.
Remove the
mark requirement. Why not have it so that it gains a benefit if you have a
mark (say, produces a massive amount of
quanta?), otherwise it's just useful as cheap damage?
