Attacks that cost 0 Energy deal 4 additional damage.
It affects any 0 cost Attack cards, regardless of class. As a result, cards like Anger, Claw, or Flurry of Blows when obtained from Prismatic Shard do work with this relic.
Shivs are the most obvious use for this relic, being a passive and sizable buff, however it applies to natural 0-cost cards like Neutralize as well.
Bullet Time reduces all card costs in hand to 0 for the turn, giving ALL attacks 4 extra damage.
Setup and Forethought reduce the cost of one card to 0 until played, which can be used on an attack to buff it. (In the case of Forethought+, can be used on your entire hand)
Snecko Eye may reduce cost to 0 when you draw an attack.
Attack Potion sets the cost of the given attack to 0 for the turn.
Liquid Memories sets the cost of the retrieved card to 0 for the turn.