NWScript:

void main()
{
    // Sometimes spawn monster during rest
    // An object to hold the last player who rested
    object oPlayer = GetLastPCRested();
    // Check to see whether resting has started or is over
    int nRestTurn = GetLocalInt(oPlayer, "nRestTurns");
    SetLocalInt(oPlayer, "nRestTurns", nRestTurn + 1);
    nRestTurn = GetLocalInt(oPlayer, "nRestTurns");
    if (nRestTurn == 1) {
        // Determine whether rest will be peaceful or not
        int nRestType = d6();
        int nEnemyType = d3();
        object oEnemy;
        // A string to hold the creature's tag
        string sCreature;
        // Set a location for where the PC is
        location lHere = GetLocation(oPlayer);
        // Set the level of the monster to spawn
        int iLevel = 0;
        iLevel += GetLevelByPosition(1, oPlayer);
        iLevel += GetLevelByPosition(2, oPlayer);
        iLevel += GetLevelByPosition(3, oPlayer);
        // A roll of 4 causes the monster to be even harder
        if (nRestType == 4) {
            iLevel++;
            nRestType = 3;
        }
        // Script only has 15 places for monster levels
        if (iLevel > 15) {
            iLevel = 15;
        }
        // Bad luck, a monster is coming
        if (nRestType == 3)  {
            // Decide which monster to spawn, based on CR
            switch (iLevel) {
                case 1:
                    if (nEnemyType == 1)
                        sCreature = "NW_ORCA";
                    else if (nEnemyType == 2)
                        sCreature = "NW_WOLF";
                    else
                        sCreature = "NW_ZOMBIE01";
                    break;
                case 2:
                    if (nEnemyType == 1)
                        sCreature = "NW_BEARBLCK";
                    else if (nEnemyType == 2)
                        sCreature = "NW_BUGBEARA";
                    else
                        sCreature = "NW_KRENSHAR";
                    break;
                case 3:
                    if (nEnemyType == 1)
                        sCreature = "NW_MEPAIR";
                    else if (nEnemyType == 2)
                        sCreature = "NW_BTLBOMB";
                    else
                        sCreature = "NW_DIREBADG";
                    break;
                case 4:
                    if (nEnemyType == 1)
                        sCreature = "NW_GHAST";
                    else if (nEnemyType == 2)
                        sCreature = "NW_AIR";
                    else
                        sCreature = "NW_MINOTAUR";
                    break;
                case 5:
                    if (nEnemyType == 1)
                        sCreature = "NW_OGREMAGE01";
                    else if (nEnemyType == 2)
                        sCreature = "NW_BUGBEARA";
                    else
                        sCreature = "NW_KRENSHAR";
                    break;
                case 6:
                    if (nEnemyType == 1)
                        sCreature = "NW_OGRECHIEF01";
                    else if (nEnemyType == 2)
                        sCreature = "NW_TROLL";
                    else
                        sCreature = "NW_DEVOUR";
                    break;
                case 7:
                    if (nEnemyType == 1)
                        sCreature = "NW_SLAADBL";
                    else if (nEnemyType == 2)
                        sCreature = "NW_UMBERHULK";
                    else
                        sCreature = "NW_MUMMY";
                    break;
                case 8:
                    if (nEnemyType == 1)
                        sCreature = "NW_GOLFLESH";
                    else if (nEnemyType == 2)
                        sCreature = "NW_ETTIN";
                    else
                        sCreature = "NW_TROLLCHIEF";
                    break;
                case 9:
                    if (nEnemyType == 1)
                        sCreature = "NW_BEARDIRE";
                    else if (nEnemyType == 2)
                        sCreature = "NW_MINWIZ";
                    else
                        sCreature = "NW_RAKSHASA";
                    break;
                case 10:
                    if (nEnemyType == 1)
                        sCreature = "NW_GNTFROST";
                    else if (nEnemyType == 2)
                        sCreature = "nw_bugbearboss";
                    else
                        sCreature = "NW_GNTHILL";
                    break;
                case 11:
                    if (nEnemyType == 1)
                        sCreature = "NW_GOBLINBOSS";
                    else if (nEnemyType == 2)
                        sCreature = "NW_BATTDEVOUR";
                    else
                        sCreature = "NW_REVENANT001";
                    break;
                case 12:
                    if (nEnemyType == 1)
                        sCreature = "NW_GolClay";
                    else if (nEnemyType == 2)
                        sCreature = "NW_SHGUARD";
                    else
                        sCreature = "NW_DRGBLACK001";
                    break;
                case 13:
                    if (nEnemyType == 1)
                        sCreature = "NW_SPIDERBOSS";
                    else if (nEnemyType == 2)
                        sCreature = "NW_DRGBLACK001";
                    else
                        sCreature = "NW_GNTFIRE";
                    break;
                case 14:
                    if (nEnemyType == 1)
                        sCreature = "NW_MUMMYBOSS";
                    else if (nEnemyType == 2)
                        sCreature = "drgblue002";
                    else
                        sCreature = "NW_GOLIRON";
                    break;
                case 15:
                    if (nEnemyType == 1)
                        sCreature = "nw_beetleboss";
                    else if (nEnemyType == 2)
                        sCreature = "NW_FIREGREAT";
                    else
                        sCreature = "NW_DRGGREEN002";
                    break;
            }
            // Create the enemy at the PC's location
            oEnemy = CreateObject(OBJECT_TYPE_CREATURE, sCreature, lHere, FALSE);
            // The PC alerts of danger
            AssignCommand(oPlayer, ActionSpeakString("I'm under attack!"));
            // Wait a moment so that the PC might get ready
            AssignCommand(oEnemy, ActionWait(3.0));
            // Tell the enemy to attack the player
            AssignCommand(oEnemy, ActionAttack(oPlayer));
            SetLocalInt(oPlayer, "nRestTurns", -1);
        }
    }
    if(nRestTurn != 1)
        SetLocalInt(oPlayer, "nRestTurns", 0);
}