Seismic Nova in an Ability in Death's Gambit. Abilities are special skills that can be performed depending on the Weapons you have equipped (much like Dark Souls 3). Ability use consumes Soul Energy, that you get from performing specific actions based on your Class.
Seismic Nova Effect
Smash the hammer down creating an explosion. Increases hammer damage by 10% for 15 seconds. Effect stacks 5 times.
How to unlock Seismic Nova