A random item on each level must be moved for the player to advance. The type of item may be different on each level. The difficulty in finding it has nothing to do with the level. There’s no indication of what each item is. The methods to move it varies.
As a half-apology, half-reward for the effort, an explanation can be worked into story lore near the end. Like, the world’s gods want specific items in specific locations, but they don’t talk to anyone who can move the items.
On replay, those items contain eldritch things that might fight the player.