Leviathan eff?

I saw on a youtube video of someone's deck profile that if Fabled Leviathan is negated that supposedly his effect still works. I don't think thats the case because he wouldn't have even hit the field, but I don't see any rulings on it.

If it is the case, however, Fables got even better. I'd love to be able to bait Solemn Warnings with that. XD

I'm not sure wat is written in fabled leviathan DN's version.
but, the official text is:

1 "Fabled" Tuner + 1 or more non-Tuner monster.
When this card on the field is destroyed and sent to the Graveyard, you can select up to 3 "Fabled" monsters in your Graveyard. Add them to your hand.

If the synchro summon is negated, you don't activate it's effect because it is not considered to be on the field at that time.

additional note: Scrap Dragon/LaDD and friend doesn't need to be destroyed while on the field so they'll activate their effect even if the summon was negated by solemn warning.

I know Scrap Dragon gets its effect if solemn warninged, but if the dragon was warninged can i call of the haunted/monster reborn him?

his synchro summon has been negated, hence it's never properly synchro summoned. You can't reborn/call of the haunted him.