In The Membrane, each wave = 1 day.
As an indentured servant who's signed your life away to service at The Membrane your job is to contain everything and (everyone?) that comes out of it. Every ten 10 days (10 waves) of service in the unfathomable hell of the trenches, a new sort of giant abomination arises. Some say it's something to do with the lunar cycle, but you don't care about why. You only care about how to kill these vile secretions of the Membrane.
Every 10 days you discover a unique boss. Each boss will have different phases. Some have 3 some have fewer, others more?!
The phases may include invincible stages where you must destroy the boss' minions or extensions before you can deal damage to them.
Other phases will include unforeseen attacks that you will learn only by facing them. If you collapse at The Membrane or let too many enemies escape (whereupon they will surely raid some innocent village nearby) you will be forced to start your run again.
Why?
Because the Governor is not paying you servants handsomely for you to fail. Did you not read the contract?
But here's an interesting detail. Sometimes there is a chance that, when killing them, these bosses will drop a special object. You may use this object to face that boss instantly and skip the waves that precede it.
That's good news for once.
Legends reveal other uplifting promises. The old tomes say that behind all of these bosses there is a central being. One who if you manage to destroy The Membrane will cease to exist.
No one believes this to be true, however. Since no one alive has ever met this being or force. It is said to be a mere myth made up to keep morale at The Membrane from collapsing altogether. Which is why Governor Evindar promises the bottomless gold chests of his palace to the servant who manages to bring him its skull.
The only way he'd ever be so generous would be if he himself knows it to be mere legend.
Thank you for reading!
-Gerardo