Effect: Seems to restore Grym's movement[edit source]
Currently the page says there is no known effect of Seismic Wrath. As far as I can tell, the effect is to restore Grym's movement after standing up from being knocked prone by the forge hammer.
Normally standing up from being Prone consumes all of Grym's movement, after which he ends his turn immediately. But the first time he is struck with the forge hammer, he uses Seismic Wrath. On that turn, he then breaks his usual pattern by walking toward his target and using Quake if someone is in range.
The difference in behavior compared to subsequent Forge Hammer blows is clear. On later hammer blows he can't use Seismic Wrath, so standing up from prone wastes his entire turn.