OnEntityEnter

using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using Oxide.Core;
using UnityEngine;

namespace Oxide.Plugins
{
    [Info("Radiation Zone Guard", "RustPluginGenerator", "1.0.0")]
    [Description("Warns players and applies extra damage when they enter a radiation trigger zone.")]
    public class RadiationZoneGuard : RustPlugin
    {
        private const string PermExempt = "radiationzoneguard.exempt";

        private PluginConfig config;
        private readonly Dictionary<ulong, float> lastWarn = new Dictionary<ulong, float>();

        private class PluginConfig
        {
            [JsonProperty("Extra damage per entry")]
            public float ExtraDamage { get; set; } = 5f;

            [JsonProperty("Warning cooldown in seconds")]
            public float WarningCooldown { get; set; } = 30f;
        }

        protected override void LoadDefaultConfig() => config = new PluginConfig();

        protected override void LoadConfig()
        {
            base.LoadConfig();
            try
            {
                config = Config.ReadObject<PluginConfig>();
                if (config == null) throw new Exception();
            }
            catch
            {
                PrintWarning("Configuration file is invalid, using defaults");
                LoadDefaultConfig();
            }
            SaveConfig();
        }

        protected override void SaveConfig() => Config.WriteObject(config);

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["Warning"] = "You entered a radiation zone. Get out before it burns through your gear."
            }, this);
        }

        private string Msg(string key, string userId = null, params object[] args) => string.Format(lang.GetMessage(key, this, userId), args);

        private void Init()
        {
            permission.RegisterPermission(PermExempt, this);
        }

        private object OnEntityEnter(TriggerBase triggerBase, BaseEntity ent)
        {
            var radiation = triggerBase as TriggerRadiation;
            if (radiation == null) return null;

            var player = ent as BasePlayer;
            if (player == null || !player.IsConnected || player.IsDead()) return null;
            if (permission.UserHasPermission(player.UserIDString, PermExempt)) return null;

            if (config.ExtraDamage > 0f)
            {
                player.Hurt(config.ExtraDamage, Rust.DamageType.Radiation);
            }

            float now = UnityEngine.Time.realtimeSinceStartup;
            float last;
            if (lastWarn.TryGetValue(player.userID.Get(), out last) && now - last < config.WarningCooldown) return null;

            lastWarn[player.userID.Get()] = now;
            player.ChatMessage(Msg("Warning", player.UserIDString));
            return null;
        }
    }
}