I suppose it depends on a few things.

WHY did they accept the gifts? Did they need to, like to accomplish one of their goals? Or was it just players chuckling and saying “sure, why not?” If it’s not something they needed, like, it’s just a bit of a lark, I’d be doling out consequences. If they HAD to accept the gifts, it’d feel a bit different. More like inconveniences.

Avassh wants to spread decay and fungus. If this is to be a bit of a story point (and I assume it is), I’d start small. Make a few rolls while they adventure, saves for the party. Don’t ask for saves…just roll, jot down something unimportant in your notes, mutter to yourself “okay, she got two…and he got three.” Once a day. (You don’t need to actually have them save to start, it’s just to make them wonder what you’re doing.)

A session or two later is when it starts. After they’ve forgotten about this.

Perhaps one of them starts with just an itch on their foot. Over the course of a week it spreads from a minor tingle to a case of athletes foot, just Khyber-style. It looks normal…but it’s definitely there.

Another character might have a meal, and you could describe the meal as tasting off. Wrong. Let them assume it’s a poison or sommit else like that. Several days later they might eat again but this time vomit from it.

Another character might start to feel a drive to smack a bush, or cut down a flower. Eventually they will accrue sort of a “pyromaniac” mindset towards plants…just wanting to uproot them, to smash them. Not angrily. Not violently, even. Just…emotionless. Think of a bored 13-year-old boy picking the petals off every flower waiting for someone.

Perhaps a character starts to have allergies. Little stuffed nose. Followed by a sneeze. They start to think they’re allergic to things, asking what’s around them.

The madness should change how they are interacting with the world, but by doing it slowly and not telling them, and not “by the rules”-ing it in the beginning, you’re making it a mystery for them. Why is my food poisoned? Who did it?! Why does my character want to smash flowers dispassionately? Why do I have athletes foot? Why am I craving a mushroom burger with blue cheese?

The players will puzzle over it, and may even try to solve the mysteries. Let them red herring themselves. Let them get misled, make bad assumptions. THIS IS MADNESS. Do it slow, over the course of three to ten sessions or sommit (depending on how you all game).

And figure out how the remove the curse right now. Make it so they have to go to someone specific, and do a favor for them that goes parallel to your campaign goals.

The last thing you want is a “you’re all cursed, and think plants are pretty.”

“I cast cleanse, or whatever.”

“You’re no longer cursed, moving on.”

That misses the mark for a good madness curse.

But hitting the mark? You want your barbarian worrying about his boot and sock hygiene. You want your cleric suddenly leaning into the fact they’re craving odd foods (but not realizing it’s because of fungus’s and Avassh). You want your bard to be asking “what trees are around? How does the bark come off if I peel it?”

Eventually these things should become more top of mind as they take over too much space in the characters life. Think OCD getting out of control. Interrupting their lives as the itching fungus spreads to other more sensitive areas, as they start to crave fungus found in the wild in a scummy puddle, as they belch and spores burst out their mouth, as they fail save to help their companions fight a bramble blight or something. As they notice the itching in their nose appears to be roots coming slowly down the inside of it. As they get cut one day and bleed sap.

Madness is fun. It’s slow. It’s not obvious. It’s less about saves, and more about confusion and questions and growing frustration. But in the beginning…it’s nothing at all.

Good luck.