Two people with arms died, and were buried on top of each other. 

Because it is actually Stalmaster

Ebay

He took something from the warriors he defeated... namely their arms.

He is Nocturn Reborn.

Kulta tried to be a smart guy and commanded him to >Retrieve arms from coffin. HE OBEYED.

he had four weapons and went I NEED TO USE THEM ALL

Two people with arms died, and were buried on top of each other. 

How did they fuse into one skeleton with four arms?

Ebay

Ebay exists on Okoto? Ekimubay?

He is Nocturn Reborn.

Nocturn?...Reborn? Alright, somebody create a 'Diurn' with two legs and four legs. If we have a character whose name is derived from nocturnal we need a character whose name is derived from diurnal. 

Kulta tried to be a smart guy and commanded him to >Retrieve arms from coffin. HE OBEYED.

But what if he already had four arms?

he had four weapons and went I NEED TO USE THEM ALL

Sheer force of will is enough to spontaneously grow two extra arms?

 

Two people with arms died, and were buried on top of each other. 

 

How did they fuse into one skeleton with four arms?

Kulta has the power of bone rearranging - if he can arrange old bones into scorpions, surely he can rearrange two skeletons into a 4-armed monster. 

Well, if he can do that, why not create a giant bone-mech and squish those pesky Toa with it?

Sheer force of will is enough to spontaneously grow two extra arms?

YES. SKULL SLICER BELIEVED. AND IN HIS BELIEF, HE ASCENDED TO FOUR ARM MASTER RACE.

Kulta tried to be a smart guy and commanded him to >Retrieve arms from coffin. HE OBEYED.

 

But what if he already had four arms?

Then this question is rather pointless.

 

Sheer force of will is enough to spontaneously grow two extra arms?

YES. SKULL SLICER BELIEVED. AND IN HIS BELIEF, HE ASCENDED TO FOUR ARM MASTER RACE.

 

 

Kulta tried to be a smart guy and commanded him to >Retrieve arms from coffin. HE OBEYED.

 

But what if he already had four arms?

Then this question is rather pointless.

 

What if he wants six arms?

Well, if he can do that, why not create a giant bone-mech and squish those pesky Toa with it?

Besides, how do you know that mechs exist on Okoto? Maybe he never saw one.