In a small bowl, combine the mustard seed, dry mustard, and cider vinegar. Cover the bowl with plastic wrap and let the mustard mixture stand at room temperature for 3 hours.
In a small saucepan, combine the dark beer, garlic, brown sugar, salt, ginger, and allspice. Stir in the mustard mixture. Over moderate heat, bring the mixture to a boil; reduce the heat to low and simmer for 5 minutes, stirring occasionally.
Spoon the mustard into a hot, clean canning jar and seal tightly. Let the mustard cool to room temperature. Store in a cool, dark place and in the refrigerator after opening. Makes about 2 cups.