"My knife is indeed very bad."
Translation:Kisu changu ndicho kibaya sana.
Ndicho is the verb. It's the emphatic form of ni and takes class suffixes.
Kisu ni kibaya = the knife is bad
Kisu ndicho kibaya = the knife is indeed bad / the knife is the bad one
I think ni originally comes from ndi- and lost the endings and became pronounced as ni because it's such a common word.