In a large pitcher, combine tequila, triple sec, blue curacao, lime juice, and agave nectar.
Stir in the cold water.
Cover and refrigerate up to 3 days.
Stir and taste before serving, adjusting lime or agave if needed.
Fill glasses with ice, pour margarita over, and garnish.
Notes
For a smoking cauldron effect, drop a small piece of food-grade dry ice into the pitcher just before serving. Never touch dry ice with bare hands and keep it out of glasses guests will drink from.
Keyword halloween batch margarita, halloween tequila cocktails, make ahead margaritas for a crowd