I don't know what is happening but the game is saying the root cause is this mod:

Here is the error I'm getting:
System.Collections.Generic.KeyNotFoundException: ThoriumRework.BossOverrides
at ThoriumRework.BossStatScalingWithCalamity.PostAI(NPC npc) in ThoriumRework\BossStatScalingWithCalamity.cs:line 161
at Terraria.ModLoader.NPCLoader.PostAI(NPC npc) in tModLoader\Terraria\ModLoader\NPCLoader.cs:line 339
at DMD<System.Void Terraria.NPC:UpdateNPC_Inner(System.Int32)>(NPC this, Int32 i)
at Hook<System.Void SOTS.SOTSDetours::NPC_UpdateNPC_Inner(Terraria.On_NPC+orig_UpdateNPC_Inner,Terraria.NPC,System.Int32)>(NPC , Int32 )
at SyncProxy<System.Void Terraria.NPC:UpdateNPC_Inner(System.Int32)>(NPC , Int32 )
at DMD<System.Void Terraria.NPC:UpdateNPC(System.Int32)>(NPC this, Int32 i)
at CatalystMod.CatalystDetours.NPC_UpdateNPC(orig_UpdateNPC orig, NPC self, Int32 i)
at Hook<System.Void CatalystMod.CatalystDetours::NPC_UpdateNPC(Terraria.On_NPC+orig_UpdateNPC,Terraria.NPC,System.Int32)>(NPC , Int32 )
at SyncProxy<System.Void Terraria.NPC:UpdateNPC(System.Int32)>(NPC , Int32 )
at DMD<DMD<>?34221150::Terraria.Main::DoUpdateInWorld>(Main this, Stopwatch sw)
at SyncProxy<System.Void Terraria.Main:DoUpdateInWorld(System.Diagnostics.Stopwatch)>(Main , Stopwatch )
at DMD<DMD<>?27206532::Terraria.Main::DoUpdate>(Main this, GameTime& gameTime)
at DragonLens.Content.Tools.Gameplay.FastForwardSystem.UpdateExtraTimes(orig_DoUpdate orig, Main self, GameTime& gameTime)
at Hook<System.Void DragonLens.Content.Tools.Gameplay.FastForwardSystem::UpdateExtraTimes(Terraria.On_Main+orig_DoUpdate,Terraria.Main,Microsoft.Xna.Framework.GameTime&)>(Main , GameTime& )
at HighFPSSupport.TickRateModifier.OnDoUpdate(orig_DoUpdate orig, Main self, GameTime& gameTime)
at Hook<System.Void HighFPSSupport.TickRateModifier::OnDoUpdate(Terraria.On_Main+orig_DoUpdate,Terraria.Main,Microsoft.Xna.Framework.GameTime&)>(Main , GameTime& )
at RealisticSky.Content.MainMenuSkyUpdater.UpdateSky(orig_DoUpdate orig, Main self, GameTime& gameTime)
at Hook<System.Void RealisticSky.Content.MainMenuSkyUpdater::UpdateSky(Terraria.On_Main+orig_DoUpdate,Terraria.Main,Microsoft.Xna.Framework.GameTime&)>(Main , GameTime& )
at SyncProxy<System.Void Terraria.Main:DoUpdate(Microsoft.Xna.Framework.GameTime&)>(Main , GameTime& )
at DMD<System.Void Terraria.Main:Update(Microsoft.Xna.Framework.GameTime)>(Main this, GameTime gameTime)
at Hook<System.Void ThoriumMod.UI.ThoriumInterfaceResources::OnResolutionChanged(Terraria.On_Main+orig_Update,Terraria.Main,Microsoft.Xna.Framework.GameTime)>(Main , GameTime )
at Hook<System.Void BTitles.BiomeTitlesMod::Update(Terraria.On_Main+orig_Update,Terraria.Main,Microsoft.Xna.Framework.GameTime)>(Main , GameTime )
at Hook<System.Void DragonLens.Content.GUI.Tooltip::Reset(Terraria.On_Main+orig_Update,Terraria.Main,Microsoft.Xna.Framework.GameTime)>(Main , GameTime )
at SyncProxy<System.Void Terraria.Main:Update(Microsoft.Xna.Framework.GameTime)>(Main , GameTime )
at Microsoft.Xna.Framework.Game.Tick()
at Microsoft.Xna.Framework.Game.RunLoop()
at Microsoft.Xna.Framework.Game.Run()
at Terraria.Program.RunGame()
at Terraria.Program.LaunchGame_(Boolean isServer)
at Terraria.Program.LaunchGame(String[] args, Boolean monoArgs)
at Terraria.MonoLaunch.Main_End(String[] args)
at Terraria.MonoLaunch.<>c__DisplayClass1_0.<Main>b__1()

I am playing with the Infernal Eclipse of Ragnarok mod pack enabled, which includes Ragnarok and the Bard/Healer mods.