I have no suggestions as I am trying to get the hang of Kathril myself but I do like what you are doing with the deck! Looks definitely fun to play against. I must admit when I read the title, I thought it would be Nightmare tribal.

Thanks! Yeah, I can kind of see that now, lol. The thing to look out for with Kathril is beware of graveyard hate. When playing, don't dump your bomb creatures into your graveyard until you are ready to play Kathril.

Stitcher's Supplier instead of Crystalline Giant , It's cheaper to play and doesn't care about being removed, chances are you will dump something with a keyword immediately making it better.

Valentin, Dean of the Vein  ^Flip instead of Nullpriest of Oblivion , cheaper to play with the same keywords while also stopping creatures from staying in the graveyard and potentially making your blockers. You may even play the flip side in this deck so that's added value.

Gurmag Swiftwing is worth considering since you don't have many first strikers.

Tortured Existence is amazing in this deck especially if you decide to run cards with cycling like Pale Recluse or Archfiend of Ifnir

Unearth is just good

Thanks for the suggestions! I like the Valentin, Dean of the Vein  ^Flip and was thinking of adding it in before you suggested it, but it may be a swap in for Fiend Artisan as it isn't working as intended. Tortured Existence is awesome and will work its way in. Still deciding on if Crystalline Giant is worth having in the deck, so I may make that change or something similar.