These ideas have been sitting in my head for awhile and are pretty rough ideas I personally like for a fiend themed boss zone.

I had a basic idea of having a boss area based on and for fiends.
I love the corporate structure the devilites have and consider it a interesting idea that a spiral scout intercepted a communication within the devilites corporate ladder that there was a major business deal going down that some unknown force wants the knights off the Cradle. (Now this could be a deal between the Gremlins and the Devilites but I am not sure if they have any relations to each other so the "unknown force" can be something that three rings can put together)

Anyway the main idea is that knights have to be dispatched the newly discovered Head Corporate center the Devilites run their entire company out of. I was thinking it would be a walk down the city streets that look in almost perfect condition with even some vehicles that some of the little demons drive. As you're fighting out in the open to reach the front gate of the HQ.
These streets will have all sorts of things like propaganda banners that read things like "A working devilite is a productive devilite" "Longer hours lead to greater achievements." or a banner that shows the spiral order's symbol with a circle around it and a slash in the middle, and the banner would be like "Even the spiral order cannot compete to our productivity!"
Neighborhood watch posters that would be like "If you see a kinght report it immediately. Get a bonus on your paycheck for doing your part!"
The best way to move around is a more wide exploration area then the normal more linear type floors for the first level. giving back allys and things that you can use to avoid the main roads. And try to keep out of sight of police officers and pedestrians.

There would also be new types of devilites such as...

Pedestrian Devilites: They are more in number, have basic melee attacks and don't really attack unless provoked. Little health as well.
Whistle Blowers: These Devilites are slightly different then the pedestrians as they wear the neighborhood watch uniforms. They also have a club, like a nightstick for a basic weapon. If you don't kill it right away you will end up getting swarmed by pedestrians in an area when they blow their whistles.
As well as police officers arriving in a short time.

Police Devilite: These guys are packing heat either have guns or shadow attacks with elements to it (like the normal workers based on their stratum)
They attack harder then the other two, more in terms of power on a normal worker.
Swat Devilite: These guys will get called in with the police and will be guarding the front door of the HQ with the Head of the corporate police force.
In terms of attack power they are about the same with the police devilites other then the fact they have shields that can break but withstand a good amount of attacks before doing so. If you break their shield they downgrade to police devilites
The head of the corporate police force will be a larger devilite armed with all sorts of weapons and takes a good bit to kill. Possibly a mini-boss.

These would be just for the first floor there may be pedestrians and whistle blowers in the start of the second floor but once inside the HQ its all employees.

New Employee types

Janitorial Staff: These guys are actually neutral to you. and you can even talk to one at the start of the HQ level. I was thinking they will be wearing janitor suits like you see on cleaning stuff for major office buildings, including a hat, and they have a cleaning tool (broom, mop, etc)
The first group will tell you at the start that they do not get paid enough to even care what goes on inside. As long as the knights don't cause a major mess. (What I was thinking of was there can be destructible computers, cubicles, office equipment etc. And when you do destroy a good amount of items, you will get swarmed with additional enemy's called Overworked janitor. That has some AoE attacks using their cleaning tools to fight you)

Under management: They guys just command devilites to attack you and do nothing else. They may promote a dev to a yesman and don't do much unless you kill their workforce before you kill them. Causing them to attack you or run away to middle management if there is one in the area, and becoming a Managers Yesman. They can heal the middle manager and buff attacks of other under managers or employees of an area.

Middle Management: They can praise, demote (if the under manager loses enough staff) or command under management. They themselves will not lift a finger unless they have to. And will pass on all the work to the under mangers and their staff. So if in a situation that a Middle manager is in a room, and is under attack he will tell under management to defend him with their life. If their staff dies and there is under managers, a under manager will take over his position and get demoted to a normal employee.

On the last floor (highest floor) You will end up in the board room after fighting some upper management and corporate security forces. There is where the main boss fight happens, where you have to fight the chief staff head on. Such as the CIO (Chief Information Officer) COO (Chief Operations Officer) CFO (Chief Financial Officer) CTO (Chief Technology Officer) and any other Chief positions wanted. (Was thinking one per elemental type if possible) Then after fighting most of the Executive board the door to the CEOs office opens up and he walks in snickering saying like "Well, well knights, I see you had a nice meeting with my executive staff. However it is rather... Unfortunate that the Chairman has stepped out for the evening. He has left me the task of filling you in with the rest of our business plans. Right after I kill you."

Battle starts in his large roomy office of death.

That is all i really have for now, like I said its all rough ideas but I hope people like it and pick up on it.