Yamarāja is an ancient being who’s even older than Ekimu and Makuta. He was brought in to Okoto by his creator so he could guide the souls of the dead into heaven or hell, thus giving him godlike status.
In the beginning he did as he was suppose to do, guiding the souls to where they belonged after death, but as times went on, Yamarāja became obsessed with death and started to kill Okotoans just so he could set their souls free from their bodies.
When his creator found out about this, he became furious and made sure that Yamarāja would never ever be free. Cursing him with immortality and taking away his powers.
700 years later, stll wanting to be set free, Yamarāja stumbles upon Ekimu and Makuta, the 2 legendary brothers of Okoto. He tells them about how he was wrongfully punished by the creator, and that he only wanted to set souls free.
Ekimu tells him that he will not help anyone who dares to say against the creator and walks away. Makuta on the other hand wants to help him, but Ekimu must not know about this.
Yamarāja and Makuta agrees to meet in a secret place where Makuta would make him a new mask. Yamarāja thinks he will get his old power back, but Makuta tells him that he can’t choose his mask power and that Makuta would find him a good power. Without hesitation he says against Makuta.
“The only thing I want is to be set free aswell, give me the power to free myself from this body”
Makuta laughs and tells him that won’t happen. Instead Makuta puts a mask on Yamarāja which transforms him into a monstreous being with a pet of his own.
you’re under my command now, Makuta tells him. Now you’re Yamarāja The god of death.
“You won’t be set free, nor will you live, you’re just a monster in limbo between life and death”
Enraged and with a new power he unleashes a black aura that explodes over a big radius killing everything inside it except for Makuta.
Almost all of his memory is gone because of the transformation and Makuta wiping some of it away, the only thing he can remember is “Memento mori, remember your death”
After the fall of Ekimu and Makuta due to the grat cataclysm, Yamarāja is told by Makuta’s soul that he will have to kill the 6 heroes the prophecy told about, if hes other pawns failed. Kulta and Umarak failed in what they were told, and after Umarak the destroyer’s defeat at the hands of the Toa, Yamarāja appears.
“Remember your life, remember your death, Toa. Memento mori is upon you”
And now to the actual pictures of my moc, got carried a bit away when I wrote his lore 
I’ll take some better pictures later, when I set up some sort of studio.
Any feedback appreciated.