Your ecosystem is the dungeon. You give up your ability to wildshape into beasts to instead wildshape into monsters. Other druids tend to ostracize circle of the dungeon druids seeing them as disconnected with nature. You can have a home dungeon or simply protect dungeons and monsters in general. 

Level 2: Monstrous Wild Shape

Starting at level 2 you get a list of 10 different monsters you can turn into on a bonus action. However you do not get to pick which one you transform into. Instead you roll a d10 and must transform into that creature until you run out of it's hit points or wild shape back to your normal form. If you land on a monster with a challenge rating above your maximum transformation cr your use of wildshape is wasted. Your starting maximum wildshape cr is equal to your druid level divided by 3 rounded down. The starting list is: 

1. Animated armor
2. Ghoul
3. Goblin boss
4. Modrone
5. Hippogriff
6. Zombie
7. Skeleton
8. Warhorse skeleton
9. Myconid adult
10. Young white dragon

By taking this subclass you can no longer wildshape into beasts. When you wildshape you keep your intelligence, wisdom, and charisma scores while adopting the stats and abilities of the monster you wildshaped into. If the creature you wildshaped into has spellcasting then you can still cast spells. 

Creature Acquisition

Starting at level 2 you can replace monsters on your list with those you come in contact with. If you look into a creatures eyes you can on a bonus action roll a d10 and replace the monster it lands on with this creature. Alternatively on an action if you eat part of the creature you can choose which monster to replace it with. If the monster is deemed by the dm to be unsafe to eat you must make a constitution saving throw equal to it's challenge rating times 5, if you fail you become sick and gain one level of exhaustion. You still get to add it to your monster list though. 

For a monster to be eligible for this feature it must have a challenge rating and not be something a druid can normally turn into. You can add monsters that have too high of a challenge rating for you to turn into. 

In addition you can change the order of your list on a long rest. 

Level 3: Monster Communication

Starting at level 3 you can communicate with monsters on your monster list. If the monster can speak you speak its language otherwise you can tell it's feelings. When you try to communicate with a monster you can use persuasion on monsters that talk and animal handling on ones that don't. 

Level 6: Wildshape Options

Starting at level 6 when you wildshape you can choose to roll 2 d10s and choose which one to take. You can also bypass the dice roll and choose which creature to turn into but if you do so you lose that creature from your monster list. 

Level 10: Shared Transformation

Starting at level 10 you can expend a use of your wildshape to transform another creature. If the creature is unwilling they must fail a save of their highest stat against your spellcasting dc. The creature being transformed is the one that gets to pick which creature they become. They can use the wildshape options feature to roll multiple d10s but not to bypass the roll. 

Level 10: Special Wildshape

Starting at level 10 you get a special wildshape that can be a cr up to half your druid level. Your starting special wildshape is chosen by a roll from this table: 
Barbed devil
Barlgura
Beholder zombie
Bulette
Cambion
Drow elite warrior
Flesh golem
Gladiator
Gorgon
Green hag (in coven)
Half-red dragon veteran
Hill giant
Mezzoloth
Night hag
Otyugh
Red slaad
Revenant
Roper
Sahuagin baron
Salamander
Shambling mound
Troll
Umber hulk
Unicorn
Vampire spawn
Werebear
Wraith
Xorn
Young remorhaz

You either roll a d29 or if you don't have one roll a percentile dice and take the modulo of it by 29. You can replace this special wildshape by eating part of an eligible creature. The same potential penalties for being sick apply as in the wildshape acquisition feature. 

Transforming into this special wildshape requires you have full spell slots expend them all to do this. This does not use your normal wildshape uses. 

Level 14: Beholding Stand

Starting at level 14 when you would be brought down to zero hitpoints roll a d20. On a 20 you wildshape into a beholder. On a one you wildshape into a beholder that is aggressive toward your allies. Either way this lasts for one minute. Your beholder form takes the damage of the attack that would have brought you to zero hitpoints.