<button class="muchFancy" onclick="buySword()"> Buy a fucking Sword </button>

var Swords = 0;

function autoSoul(number) {
    souls = souls + number;
    document.getElementById('souls').innerHTML = souls;
}

function buySword() {
   var swordCost = Math.floor(30 * Math.pow(1.25, Swords));
   if (souls >= swordCost) {
        Swords = Swords + 1;
        souls = souls - swordCost;
        document.getElementById("Swords").innerHTML = Swords;
        document.getElementById("souls").innerHTML  = souls;
    };
    var nextSword  = Math.floor(30 * Math.pow(1.25, Swords));
    document.getElementById('swordCost').innerHTML = nextSword;
};

window.setInterval(function(){
autoSoul(Swords);
}, 1000);

<button class="muchFancy" id="muchFancy">Buy a fucking Sword</button><br>
Swords: <div id="Swords">1</div>
souls: <div id="souls">150</div>
Cost: <div id="swordCost">5</div>

var Swords = 1;
var souls = 150;

function autoSoul(number) {
    souls = souls + number;
    document.getElementById('souls').innerHTML = souls;
}

function buySword() {
    var swordCost = Math.floor(30 * Math.pow(1.25, Swords));
    if (souls >= swordCost) {
        Swords = Swords + 1;
        souls = souls - swordCost;
        document.getElementById("Swords").innerHTML = Swords;
        document.getElementById("souls").innerHTML = souls;
    }
    var nextSword = Math.floor(30 * Math.pow(1.25, Swords));
    document.getElementById('swordCost').innerHTML = nextSword;
}

window.setInterval(function () {
    autoSoul(Swords);
}, 1000);

document.getElementById("muchFancy").addEventListener("click", function(){
    buySword();
});