For example:

[Go investigate] or [turn lights back on]

[turn lights back on]:
*screaming stops*

'Huh.'

[Go investigate]:
{if seen turn lights back on:Nothing to investigate anymore}{else:You were crushed by a stampede of humans}

(because if it's game over and its a passage youll still be able to click other links, since its a passage but if it's a section then if the lights are on and theres nothing to investigate you wont be able to go back)

or do you just do this:

[[Go investigate]]:
{if seen turn lights back on:Nothing to investigate anymore<br>[[Go Back]]}{else:You were crushed by a stampede of humans}

also (not sure if its me) but when i add html to it it just says: NULL. Is there a way to fix this

Hmm… I think you might need to use javascript to do that.

[Go investigate] or [turn lights back on]

[turn lights back on]:
*screaming stops*

'Huh.'

[Go investigate]:
    if (!squiffy.story.seen('turn lights back on')) {
      squiffy.story.go('stampede')
    }

{if seen turn lights back on:Nothing to investigate anymore}

[[stampede]]:
You were crushed by a stampede of humans.