Description of feature
In creative mode, I love playing around with the wither storm mod. Having mobs (try to) run away, tinkering with commands and settings, etc. I feel that their should be a potion (specifically splash potion) of wither sickness. I would love to see a crowd of pigs with Wither Sickness to see how they react to it, just for science ig.

also throwing this at your friends would be funny lol.

Give them the effect using commands /effect give @e/@A witherstormmod:wither_sickness

regular potion:
/give @p potion{display:{Name:'{"text":"Potion of Wither Sickness","italic":false}'},CustomPotionEffects:[{Id:255b,Amplifier:0b,Duration:19999980}],CustomPotionColor:9569279} 1

splash potion:
/give @p splash_potion{display:{Name:'{"text":"Splash Potion of Wither Sickness","italic":false}'},CustomPotionEffects:[{Id:255b,Amplifier:0b,Duration:19999980}],CustomPotionColor:9569279} 1

lingering potion:
/give @p lingering_potion{display:{Name:'{"text":"Lingering Potion of Wither Sickness","italic":false}'},CustomPotionEffects:[{Id:255b,Amplifier:0b,Duration:19999980}],CustomPotionColor:9569279} 1

Wither sickness isn't meant to be a potion. You can use these commands or a potion generator online to get a potion however.