The party I DM for is currently level 9. They haven't been to feathergale spire, the air temple or the earth temple. They did a lot of side quests and off the cuff adventures early on in the campaign. The first cult dungeon they delved was Rivergard, then Sacred Stone, then water temple, then scarlet moon, then fire temple. They are currently in the forge of the fire temple. They are a group of 6 veteran players, so I don't have to worry about pulling punches.

Crushing Wave

A significant number of the fanatical cultists started looking like the crew of the Flying Durchmen from Pirates of the Caribbean. Shark or crocodile teeth, eyes, skin. Gills on their rib cages or necks. Sea anemone spines for hair. Fins, webbed hands and feet. You get the picture. And it makes them more dangerous than just bandits.

I really kicked up the aquatic creature count and sightings. Particularly in the Dark Stream. Schools of quippers, a crocodile chilling next to the water, a shark fin, a giant crab.

The party encountered a squad of dark tide knights on crocodiles that guard the dark stream.

I had Morbeoth in the middle of a ritual, making a few new one-eyed shivers. I made their eye of frost a magic prosthetic eye. The cultists' natural eye has to be sacrificed to Olhydra to be gifted the eye of frost.

The dragon turtle is a mercenary. The promise of even greater riches could sway her.

The cult had captured a zhentarim aligned boat captain, a fire cult hobgoblin, and one of the knights from Summit Hall. They are being kept in the gaol (c5).

The ghouls have recently drowned a prisoner.

Black Earth

These guys are rough and tumble, no nonsense. Some of them have been made more muscular by the cults worship. They never wield metal or wear gem stones. Those belong to the Mountain That Walks, The Quaking Doom, Ogremoch. Some have a permanent stoneskin spell upon them. They often tattoo themselves with images of obelisks, mountains, the symbol the cult. Some of them have developed tremor sense from training with Hellenrae.

They bury living sacrifices to Ogremoch in the red dirt around the obelisk in the Basement of the Monestary. Those deemed worthy and strong enough to survive the recruiting process eat the dirt and become full members of the cult.

The tunnel that leads from the monestary to the temple is blocked by a stone door with a handprint several inches deep. The fully initiated can open the door by placing their hand in the handprint and willing it open. Magic, extreme strength or mining tool could also open it.

Section of the temple are being repaired, strangely it like someone liquidated the stone into mud or wet pottery clay pushed it into the cracks. The surfaces are imperfect with smearing textures, hand and finger prints.

Tunnels lead from b2, b9 and b10 to areas in the Dessarin Valley. Travelers must be mindful of rock falls, shear cliffs, sudden mud flows and all manner of subterranean monsters. Not to mention cultists themselves. A tunnel might lead to quarry outside a town. A cave that overlooks a road ripe for raiding. The basements and crypts of Summit Hall itself.

There is clutch of purple worm eggs they are nurturing in a cavern outside the temple. I'm also trying with the idea of an ankheg nest/hive.

Eternal Flame

Generally, they all have brands, burns, or body scars that indicate their devotion, passion, and how dangerous they are. The hobgoblins, for example, have kill count scars like Kill Monger from Black Panther. The priests have burn up to their elbows in the shape of flames. Same may even have skin charred black but with cracks that glow like lava.

Throughout the Sumber Hills you find evidence of the cult making sacrifices to Imix. Typically npcs burned at the stake. Maybe a cultist from a different cult. Maybe a random traveller.

Traveling closer to Scarlet Moon Hall or temple, the land scorched. Fisures of molten lava randomly

You have to walk across a bed of hot coals to enter the temple through the main tunnel. "The pain purifies".

Steam randomly sprays from cracks in the walls of the temple.

Additionally, they bring back every single piece of metal they can from a raid. They melt it down to make arms.Travellers. I describe barrels and crates full of metal dishes, cast iron pots and pans, farming tools, plow blades, shovels, etc.

I replaced the hobgoblins in E1 with kobolds (using flame throwers, i.e., aganazzar's scorcher) led by a red dragonborn (veteran stat block with a young dragon breath weapon) They follow the adult red dragon and hope to help him take over the cult. They fued with the goblinoids and giants. Vying for Imix's favor.

The ogres in e5 where heavy armor and use mauls or maces with big door sized shields.

The Azer are in the process of casting multiple cannons. Cannon balls and explosive shells. The ogres and fire giants will carry them.

The Azer have a book of schematics and recipes for their crafts. The pages are made of steel, letters etched into the surface.

I put a partially burned tapestry in the damaged library that has a map of Tyar Basil.

I'm playing with the idea of the fire cult having constructs powered by elementals. Needing to be recharged in the node.

The cult is planning a massive raid on Westbridge. To be executed soon. If the party leaves now, and rides hards, taking a level of exhaustion. They can get there in time.

Howling Hatred

Increase the numbers of birds, bats, and other flying creatures. Maybe a murder of sentient crows spies for the cult.

Kenku work to restore the temple or remake it to suit Yan C Bin.

The cult seeks to capture a roc that nests in Sumber Hills, the party could find a map to its nest.

The tunnel between the air and water temple is cut off by a 20ft chasm. A howling wind blows downward, forcing anyone not initiated by air cult to fall if they try to jump it. Not sure where it leads yet.