Prankster increases the priority of status moves by one. A move having its priority increased by Prankster will cause it to be blocked by Quick Guard.
Dark-type Pokémon are immune to opposing Pokémon's moves that gain priority due to Prankster, including moves called by moves that call other moves (such as Assist and Nature Power) and excluding moves that are repeated as a result of Prankster-affected Instruct or moves that occur earlier than their usual order due to Prankster-affected After You. Ally Dark-type Pokémon are still affected by the user's status moves.
Dark-type Pokémon can still bounce moves back with Magic Bounce or Magic Coat; moves that have increased priority due to Prankster which are reflected by Magic Bounce or Magic Coat can affect Dark-type Pokémon, unless the Pokémon that bounced the move with Magic Coat also has Prankster. Moves that target all Pokémon (except Perish Song and Rototiller, which cannot affect Dark-type opponents if boosted by Prankster) and moves that set traps are successful regardless of the presence of Dark-type Pokémon.
Documentation
https://bulbapedia.bulbagarden.net/wiki/Prankster_(Ability)
Relevant Code Points
More research needed.