I found the following two sentences:
あとで添削せにゃ。- I'll look it over later.
この世界にゃ魔法ってのがあるからな - There's magic on this world.
I checked the dictionary and some of the meanings of "にゃ" includes "Having to do something" and "On", which makes sense on both of those sentences.
あとで添削せにゃ - I'll (Have to) look it up later.
この世界にゃ魔法ってのがあるからな - (On) this world there's magic.
However, I'm not sure about the usage of せ + にゃ on あとで添削せにゃ
I'm also not sure about the meaning of からな at the end of この世界にゃ魔法ってのがあるからな. I assume it's a combination of から meaning from and the sentence ending particle な which has the same meaning as ね, but I'm not sure how something along the lines of "From, isn't it" would fit on the sentence.