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The Grand Thunder Bird is a boss from the Thorium mod and it actually easier than the King Slime surprisingly (Except in a sandstorm).
to fight this boss, you need too craft the Grand Flare Gun (four iron/lead bars, two talons (dropped by vultures) and four arcane dust (crafted from fallen stars) and Storm Flares (one iron bar, one talon, and one arcane dust). You only need one flare gun for unlimited but you will need to craft all the storm flares for each attempt. You can only summon this boss during the day in the desert.
--Mods Installed
Tremor
Thorium
Equipment Used (Mostly Modded stuff, yay!)
-Weapons
Gold Spear (Tremor)
Golden Bow (Ammo: Icy Arrows (Thorium))
Snowball Cannon
Birb Staff (Tremor)
-Armor
Icy Armor (Thorium)
-Accessories
Silver Spear Tip (Thorium)
Iron Shield (Tremor)
Enchanted Emerald Ring (Thorium)
Enchanted Diamond Ring (Thorium)
Warrior's Spark (Tremor)