Lovely nice, but upped one effect is way too strong, like sky said.
I think unupped cost 7, and upped cost 6 would pass. 
Randomly killing enemy creatures without regard to their stats or cost is usually a bad idea since it makes it hard to balance.
If the killing is biased so that creatures that have a lower cost, HP, or ATK are more likely targets, then I think that would be better.
(You could use any one of those or even all 3)
For example, skull shield's chance to kill a creature drops as the creature's HP rises.
That would be a good idea here too I think.
I.e. the chance for a prospective creature to die = (reciprocal of prospective creature's HP) / (sum of recprocals of all creature HPs)
E.g. if you had 5 skeletons, a mummy, and a bone dragon, then
-> sum of all reciprocal HPs = 5*(1/1) + 1/3 + (1/5) = 5.53
so each skeleton has a 1 / 5.53 chance to get killed, the mummy has a (1/3)/5.53 chance to get killed, and the dragon has a (1/5) / 5.35 chance to get killed.
That way, its still a potent CC, but players can attempt to use many small creatures as meat shields to protect their larger creatures.
This gives a sort of inverse advantage to decks that pack many small creatures.
It also encourages the use of mass CC combos and preserves the balancing mechanics that make earth and gravity useful even though their creatures have lower attack to cost ratios.
