GradiusModWorld.cs

using System.Collections.Generic;
using System.IO;
using Terraria;
using Terraria.ModLoader;
using Terraria.ModLoader.IO;

namespace ChensGradiusMod
{
    public class GradiusModWorld : ModWorld
    {
        public static bool bigcoreDowned;

        public override void Initialize()
        {
            bigcoreDowned = false;
        }

        public override void Load(TagCompound tag)
        {
            var downed = tag.GetList<string>("downed");
            bigcoreDowned = downed.Contains("bigcorecustom");
        }

        public override TagCompound Save()
        {
            var downed = new List<string>();
            if (bigcoreDowned) downed.Add("bigcorecustom");

            return new TagCompound { ["downed"] = downed };
        }

        public override void NetSend(BinaryWriter writer)
        {
            var flags = new BitsByte();
            flags[0] = bigcoreDowned;
            writer.Write(flags);
        }

        public override void NetReceive(BinaryReader reader)
        {
            BitsByte flags = reader.ReadByte();
            bigcoreDowned = flags[0];
        }

        public static bool IsBigCoreDowned() => bigcoreDowned;
    }
}