SMOOTHBOI06'S PROFILE

author=coelocanth
set the damage to a flat 1, it'll do 1hp damage and/or kill

author=coelocanth
Right, so change addstate to addState and it should work.

The red text is called a stack trace.
The first line is the actual error "TypeError: b.addstate is not a function".
The rest of it is showing all the function calls that led to that error happening. When debugging more complex problems than this, it would help understand what happened.
The underlined text shows the file and line number (they are also clickable links)

The second error about isDevToolsOpen is an incompatibility of Yanfly's plugin with MV 1.6.1 and can be solved by updating those plugins. It's mostly harmless though, it just means the wrong error is displayed on the screen when the game crashed, but you can find the right error using the dev tools as you just did.

author=coelocanth
Because you're using Yanfly plugins, it's actually easier.
Use YEP_SkillCore plugin and put this in your skill's note box:

<Post-Damage Eval>
if(b.level % 5 == 0) {
  b.addstate(1);
}
</Post-Damage Eval>

For the damage formula itself, I'd suggest this:

(b.level % 5 == 0) ? 9999 : 0

but you could also just leave it blank if you prefer.
A skill that does no damage may show a miss or "there was no effect on goblin"

Another consideration is that the "addState" function is unconditional - your level 5 death spell will kill people even if they have death resist accessories equipped.

To account for resistances, you should instead use "itemEffectAddState", like this:

<Post-Damage Eval>
if(b.level % 5 == 0) {
  this.itemEffectAddState(target, {
    code: Game_Action.EFFECT_ADD_STATE,
    dataId: 1, // dataId is the state number (0 for "normal attack" to use weapon states)
    value1: 1, // value1 is the normalized chance to apply the state (1 = 100%)
    value2: 0  // value2 is not used for states
  });
  b.addstate(1);
}
</Post-Damage Eval>

In this case it's a bit more complicated as you're building the data structure which would be in the database entry for the skill in code.

And finally for people with more of a coder mindset, I think an elegant way would be to make custom hit chances on skills.

This function in Game_Action could be overridden:

Game_Action.prototype.itemHit = function(target) {
    if (this.isPhysical()) {
        return this.item().successRate * 0.01 * this.subject().hit;
    } else {
        return this.item().successRate * 0.01;
    }
};

And since it has both the target as a an argument and the user as this.subject(), a custom hit chance could be written that takes levels into account.
Then the skill in the database could just have "add state: death 100%", but a custom hit chance in the notes like this:

<custom success rate>
b.level % 5 == 0 ? 1 : 0
</custom success rate>

In that case, the skill would miss anyone whose level isn't divisible by 5, and hit if it is (with the death state being added only to targets that were hit)

Wow! never knew i could make the skill in all those ways. Anyway i tried to use the first way with the <Post-Damage Eval>

if(b.level % 5 == 0) {

b.addstate(1);

}

</Post-Damage Eval> But i get this error screen whenever the enemy uses this skill]