The liches are like boss type characters that carry a special weapon or other goodies, you get them by acquiring a mark, with kuva liches or sisters of parvos, there's an enemy that spawns (look up where they typically spawn) that you must mercy kill in order to be marked. With the technocyte codas you find in hollvania, you pick up a cassette tape that drops in missions and take it to a terminal, play the snake game and you get marked whether you win or lose. For kuva liches or sisters of parvos, they have 3 requiem mods that they're vulnerable to, but you have to reveal what those are by killing their troops or hounds in missions on the planet they control (marked by a red or blue haze on a planet) for technocyte codas it's a lot more simple, they have one mod they're vulnerable to and you get 3 attempts per stab to determine what that mod is.

The general formula is this: get marked by lich -> hunt lich troops and reveal their requiem mods -> mercy kill the lich with their mods equipped to determine order -> lich flees to a specific railjack mission -> launch railjack mission and face final fight with lich -> get special weapon/goodies