Logicly, it does not stack. I could made an ability like that but Dark is already an amazing element.
Okay, then this is a decent card. Rather than using Explosions or destructive permanent control to wipe out the 'Eclipse' effect, you'll need either a powerful creature control (able to deal 10 damage total/kill it otherwise
), or the Lobotomize ability to remove it.
On that note, a few other things you should also fix the discrepancies between the card art and the table:
-The first "all" should be capitalized as in the picture, while the second 'all' should be the word 'and'. 'Light' should be lowercase
-"Loose" should be "Lose"
-Following from the above issue, you have what is known in grammar as a 'double negative'.
'Losing -2|-1' is the equivalent of 'gaining +2|+1', which is awkward...
-See if you can actually fit the word 'Darkness' in instead of 'dark' for the creatures part
-The '+' sign should be before the number in your table
Non-upgraded:
"Eternal Darkness: all dark creatures gain 2+|1+. while all Light creatures loose 2-|1-."
...should be changed to:
Non-upgraded (fixed):
"Endless Darkness: All darkness creatures gain +2|+1, and light creatures lose +2|+1."
