Hussar
Legend
But, if they want to kill the lich, they must destroy the phylactery, otherwise, they fail.You don't prep based on the player's goals, you prep based off the NPC's goals and whatever events occur at whatever times. "The lich wants to obtain the forbidden book" is an adventure setup, but the players have the liberty to choose to grab the book before the lich or just attempt to kill the lich directly. Its not a McGuffin because its not mandatory for the players.
Edit: or they can just ignore the lich and deal with the new undead kingdom later, preferably when they're stronger.
Does that not mean that any lich in any adventure automatically becomes a railroad?