How to make a lingering potion stay longer?

You use glow stone dust to make it longer, but I don't think you can make it last forever.

There are 2 options:

1. Make it drop a lingering potion, then use a /effect to give them the same effect, but for a long period of time (10000000 or longer). In order to do this, do /effect give @a 100000000 false
2. Make a custom potion through commands that has a near infinite length. In order to do this, do /give @p minecraft:lingering_potion{Potion:"minecraft:water",CustomPotionEffects:[{Id:"minecraft:",Duration:100000000000}],display:{Name:""}}. You can change to the effect of your choice, and set the name to whatever.

If you have a torture chamber, you could use commands to give wither to every player within an area (area of the chamber)

If you want partices, use repeating command blocks to generate particlues

Remember to use unconditional repeating command blocks