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Grimm is the son of Brother Nightingale, the evil king in the alternate dimension of Eastwind. Nightingale had ravaged all the lands of Eastwind and, with nothing left to conquer, sets his sights on Earth. Grimm and his brother Angar, tired of war, send a warning to the Flash who, along with Kid Flash and the Golden-Age Flash, stop Nightingale's army.
With Nightengale deposed, the people ask Grimm to be their king. Uninterested in ruling, Grimm takes Kid Flash's advice to "follow his path" and lets his brother take the crown. Unfortunately, Angar proves to be as a bad a ruler as his father, and Grimm is forced to kill him and take the crown anyway.
Now regretting taking Kid Flash's advice, Grimm becomes enraged when he discovers that Wally had not followed his own advice, and had taken the mantle of the Flash. Grimm decides to punish the speedster and take over the Earth just as his father had once tried to do. Grimm enlists the help of the Mirror Master and Captain Cold to trap Wally in a mirror dimension, where he is cut off from the Speed Force. Grimm then unwisely doubled crosses Mirror Master and Captain Cold, who then join forces with their foe the Flash to enter Eastwind and defeat Brother Grimm.
In a later attempt to 'replace' Wally, Grimm takes on the identity of a medical student and goes to Keystone, 'forcing' himself to fall in love with Linda Park, the Flash's pregnant wife, and takes her up to his kingdom via a giant beanstalk. Unable to climb the beanstalk as it shook violently whenever he came near it, Wally is nevertheless able to reach the top with the aid of Hawkman, who had been passing by when the beanstalk 'activated'. While Wally defeats Grimm in a head-on confrontation, Hawkman destroys the beanstalk, cutting off Eastwind from Keystone once again.