Druid Circle
"Swamp gas. Quicksand. Will o' the wisps leading ye into the deep bog. Aye. Ye shud avoid all a' that. Ye can prepare for 'em, if ye're smart aboot it. But if ye see a wee pair o' eyes peekin' out from the slime, ye best find a dirrent path. Ne'er met a wildman as crazy a' those mud-spatt'ad weirdoes."
-- Groil the Patient, ranger of Gulden Marsh.
Circle of Mud
Flora fauna, or fungi. All the diversity of life sprang from the same prehistoric soup, stirred up by the gods. Druids of the Circle of Mud trace the trail of life to its prime source, and seek to submerge themselves in the deep pool of potential power that still exists below all things. Their shapeshifting powers embrace the ever-mutable form of primordial sludge, and their understanding of the building blocks of life itself grants them unparalleled control over the very foundations of the earth.
Glorious Mud
When you choose this circle at 2nd level, you gain a small earthenware pot that generates a supply of magical mud. You can spend up to 10 minutes applying this mud to yourself, which lasts for one hour per minute spent until it dries and flakes off you. The mud persists through Wild Shape.
When you are covered in your glorious mud, you have advantage on Stealth checks when in dim light, or boggy, mucky, or similar environments. You may attempt to Hide in these environments even if you are within a creature's line of sight.
In addition, you gain resistance to fire damage when covered in your glorious mud.
Mudslide
Also at 2nd level, you learn the grease spell, which counts as a druid spell for you. When moving across the terrain effected by the grease spell, you double your movement speed and are not required to make a saving throw to retain your balance.
Primordial Soup
Starting at 6th level, you can use your Wild Shape to transform into a an Ochre Jelly. From 10th level, you can expend two uses of your Wild Shape to transform into either a Black Pudding or a Slithering Tracker (VG).
If your ooze form splits into multiple creatures, your Wild Shape is maintained until the last one drops to 0 hit points (or until its duration is otherwise exceeded). All split oozes within 10 feet of an ooze you choose flow back into this chosen creature to reform your body. Deduct the hit point totals of any oozes outside of this range from your own hit points.
Promethean Clay
At 10th level, you are able to churn up almost any landscape with ease. You may cast each of the following spells once at their lowest level without expending a spell slot: meld into stone, water walk, control water, stone shape, and wall of stone.
At 14th level, your control expands. You may also cast move earth, and either earthquake or tsunami once each at their lowest level without expending a spell slot.
You regain the use of these spells once you complete a long rest.
Additional materials, such as Xanathar's Guide to Everything, contain spells like erupting earth and watery sphere that add to the terrain control which Promethean Clay encorurages. You may wish to speak with your DM about replacing certain spells named in the feature (on an equal spell level basis) if you are drawing from XGtE or other supplemental sources.
Bubbling Biology
By 14th level, your body has permanently regressed partway back to the muds from which it sprang. You can move through spaces that fit creatures up to two sizes smaller than you without squeezing.
In addition, you gain proficiency in Constitution saving throws.
Concept by u/Pixel_Engine. Art Credit Tim Jacobus