You will need to breed Unfezant with a Farfetch'd, Galarian Farfetch'd, Skarmory, Tranquil, Pidove or Sirfetch'd that know Night Slash. Since Egg Moves can be passed down by either parent since Gen 6, juśt make sure that they are different genders and you should get Night Slash.
Unfortunately, all of the Pokemon learn Night Slash through Egg Moves as well, so we will have to do some Chain Breeding. Fortunately for us though, Galarian Farfetch'd is in the Field Egg Group. Now, you have the option to breed a FEMALE Galarian Farfetch'd with a MALE Nickit or Alolan Meowth, both of which learn Night Slash via level up at 28 and 36 respectively. Now just breed them until you get a Male Galarian Farfetch'd with Night Slash. Then Breed the Male Galarian Farfetch'd with a Female Unfezant until you get one with Night Slash.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Egg_Move
And a whole bunch of pages listing Pokemon to breed with.