opponent's deck gets 10% fatter? Seems pretty powerful.
Especially win you can pack 6 of these to make more than 50% of the opponent's deck become useless...
Higher cost/less relics might do.
What I suggest:
Unupped: 4 darkness, 2 relics
Upped: 5 darkness, 3 relics
Alternatively you can have it adding one relic to your deck as a kind of backfiring.