WARNING: be prepaired for a giant wall of text... If you want to hear my reasoning, it's down below, feel free to skip to the shadow lairs if you want to.
I think that we should get shadow lairs for some of the lesser used and unknown bosses such as Arkus, Margrel, and Razwog (Although he needs revisions because he acts too much like a Gremlin scorcher). What do these three bosses have in common? They are unique in the fact that they are either difficult or simply not worth fighting. Arkus, being part of the Spiral Order and a fellow knight, has his own mission called the The Silent Legion but it really can't be enjoyed due to the fact that his mission is not profitable, he really has been forgotten, and the only time you can actually enjoy his mission is if you have an elevator pass and time... Margrel is a recent boss being the secret boss of Moorcroft and drops 6x pages and the Black Kat hat. Margrel is unique in the fact that you can visit Moorcroft anytime, What's the catch? You need a Book of Ancient Rituals to summon Margrel. Because of this, Margrel isn't a boss you would fight normally or regularly (That is, unless you have some very good connections and luck) and once you run out of books, you have to wait until the next Kataclysm Event. (Who knows when that will come back) Note that this boss is luck based since the Book is about a 1% chance from a 1% Black kat so not everyone has fought or seen Margrel. Razwog... where do I begin... First off, you may be asking to yourself, who is Razwog? Razwog is a boss type version of a Gremlin scorcher who appears in the The Ancient Generator mission and the Crossing the Chasm mission. Why am i mentioning him? Because he essentially was forgotten after you completed the Rescue Camp, you were required to fight him at the Crossing the Chasm mission and then he's forgotten because of three reasons. First off, He is in the Rescue Camp, a place no knight really needs to come back to. (i'm not 100% sure if you can go back to the rescue camp once you've made it to Haven so correct me if i'm wrong) Second, Razwog doesn't have very much verity to him, I mean sure he looks different from any other gremlin you would see, but when it comes to combat, he's just another Gremlin Scorcher you would find in a deconstruction zone thus, his fight isn't very memorable. Third, Razwog is completely forgotten due to the other bosses outclassing him. When was the last time you heard someone mention Razwog? The first boss you would hear would be "Vanaduke", the other options being the Twins or the Jelly King, I won't forget the Snarbolax since some people fight him to get the Snarbostuffing for the guild halls.

Now that i got that out of the way...

First off, where are we going to put these bosses? We don't have specific stages for them to roam around. (examples being Vanaduke roaming in Firestorm citadel, Snarbolax in the Gloaming Wildwoods, ext.) My solution being to recycle some of the unique areas made obsolete with the missions for example, Margrel can be put in either Scarlet Fortress themed levels or Candlestick themed levels. Arkus can be put in Concrete Jungle/Dark City themed levels or Graveyards if wanted. Razwog Can be put in Deconstruction Zone type levels or Aurora Isles/Jigsaw valley levels and even Treasure Vaults! (Yes that is a joke... Gotta love those unlucky Treasure Vaults...)

Secondly, how are these bosses going to be different from the regular bosses? Arkus is going to be a Shadow and Normal Boss that inflicts stun, just like the original Arkus however, This Shadow Arkus is going to be Immune to Freeze, Stun, and Shock, have silkwings healing him, Additional health and defence, and have an offensive minion that respawn every phase. (I was thinking either 2x Trojans due to him being a Trojan or 3x Almirian Crusaders, perhaps being some of his fellow knights turned into Almirians.I would like some feedback on this). Margrel would be mostly the same, dealing poison, curse and shadow... and normal... The catch is that there will be silkwings, more health and defence, and will spawn Phantoms (3x). Razwog... Yea.... Razwog needs to be revised as a boss since he's nothing too unique however, i'm letting you guys decide on how Razwog should be implemented as it should be the community who decides has a say in certain matters and quite frankly, I have no idea on how to implement him. One suggestion I do have is that the level before Razwog would be a Deconstruction Zone and simply, you just need a key to head on to the boss. Catch? There is no healing pad at Razwog's level, you need to defeat all the gremlins in the deconstruction zone to access a healing pad before Razwog's level.

Third, Where are these levels going to be placed in the Shadow lair line? What I mean by the Shadow lair line is this, You need to defeat the Rabid Snarbolax(s) to access the Ice Queen, and so on. Shadow Margrel will require clearance from Darkfire Vanaduke, Shadow Arkus will require clearence from the Ice Queen however, Shadow Arkus isn't required to get clearance to the Red Roarmulus Twins thus, making him optional. Shadow Razwog requires clearance from the Red Roarmulus Twins and is also optional.

Fourth, What are the Token rewards and why should I spend CE on these Bosses if they are optional? Clear and simple, Shadow Margrel will be the ONLY boss to drop pages outside of the Kataclysm event (6-8x, Amount of tokens is subject to change in opinion) Currently, this being a bit of a work in progress, I have no idea what Shadow Arkus and Razwog will drop however, feel free to suggest what they drop. I was aiming for one of the Shadow lairs to have a high Crown payout however, doing that is controversial due to the amount of crowns flying around anyways.... another idea i had was that Arkus would drop 10x Primal, Grim, and Forge Sparks. (please give me feedback on this)

Fifth and finally, what would the Sanctuary Alchemy Machines give you? Every Shadow lair boss drops a material that is used to make a new end-of-game armor set. For Margrel, There would be a Nexus Cowl and Cloak, being the opposite of the Chaos cloak, giving a small defense to all Statuses and a low sword damage reduction on the cloak and a low gun damage reduction on the cowl. Arkus would give a powered up skelly suit (Please suggest name) That would give Curse and Fire resist med and the shadow defense of the regular dread skelly, it would give a damage bonus against undead and a low damage bonus on bombs and swords. For Razwog I was thinking of a Shadow lair gunner armor as the Sanctuary Alchemy Machine has none for Gunners. (Another suggestion needed)

As you can see, there are many holes in my suggestion but I want the community to propose what they would like to see, for those who cannot remember, here are the areas where I need feedback and suggestions.

The Stages for each boss
Margrel (Candlestick/Scarlet Fortress)
Razwog (Deconstruction zone/Aurora Isles-Jigsaw valley levels)
Arkus (Concrete Jungle-Dark City/Graveyards)
Arkus's minions (2x Trojans/3x Almirian Crusaders)
Razwog's Boss Revision
Arkus and Razwog's Tokens/uniqueness (Arkus dropping 10 of each Spark)
Sanctuary Alchemy Machine items (Stats, names)
Material names.