I'm sure I'm not the only one wondering why Thresh says something while grabbing Nautilus ("Hook, line and sinker")... Out of the 100+ champions, Nautilus isn't the one who is most likely to be related to Thresh... Or is he?

Nautilus' lore states : "something lurking in the muck grabbed hold of him". We don't know what grabbed him, but he got grabbed, and Thresh CAN definitely grab with death sentence.

" He isolates and toys with them, gradually eroding their sanity with his twisted, maddening humor" (Thresh's lore)
"He wandered without direction or sense of passing time in what felt like an eternal dream" (Naut's lore)
No comment about this one, I guess what Naut's endured is enough to "erode his sanity"

"Once Thresh takes an interest in a soul, he does not relent until he possesses it"... Wait, isn't Nautilus still in possession of his soul? Could have Thresh's plan, for any reason, failed? Is it because Nautilus seems still relatively sane? But then again, why did thresh join the league? He wants stronger souls, maybe in particular one he didn't catch the first time.

Now, what was Thresh doing in the water? isn't he supposed to wander Valoran? Remember his alternative skin : deep terror Thresh. Now that could be the thing that caught Naut. Even if most skins are non-canon, this one could be part of Runeterra... I go into wild guesses here, but there might be not one but two Threshes, one wandering land, one wandering sea. The lore doesn't give much on what Thresh was before becoming him, we can assume a lot of things, including this.

Now, I know this theory is a little farfetched, but is still believable... What do you guys think?

Probably more a reference to the fact that they "hook" people.

OMG FINALLY SOMEONE ELSE WHO AGREES WITH ME!!!! Ive been asking that since Thresh came out and everyone just tells me to shut up. But yes i will support you on this. I've emailed the LOL team a few times but as of yet no response.

Also Thresh's law states he takes his victims back to The Shadow Isles where he would perform his torture 'before their bodies succumbed to his grisly designs.' All i can say is ????????????????????????????????????

Sorry, but you really might be overthinking it. Nautilus became the way he did by some black sludge in ocean. I really don't see any relationship between Nautilus and Thresh other than they hook people.

I think you could be onto something here....
Thresh doesn't necessarily seem alive,
so it seems plausible a creature like himself could dwell underwater; as well as on land. His 'deep terror thresh' skin might be reflecting this possibility.

Thresh does roam across Valoran in search of souls, so that must include the sea surrounding it.

Thresh says that quote when hitting any champion

That's a cool theory. Would be sweet to see it implemented.

It's more likely Nautilus has a relation with Varus and Zed than thresh. Let me elaborate the black sludge is probably the same substance that took Varus. Also notice this from Zeds lore

The dark ninja followed the master inside. Moments later, the assembled ninjas heard Zed cry out in pain. Mysteriously, he emerged unscathed, and threw the severed head of the master at Shen's feet. Screaming in rage, Zed commanded his followers to slaughter the master's students and seize the box.

My theory is that it's the same substance that posses Varus and Nautlius was in the box.

How it ended up possing nautilus is another question. The substance probably originates from Ionia and they were trying to get rid of it for ever so they sealed it up or just chucked it in the ocean so no man can touch it. Maybe we will have more substance champions in the future.