Hello, my name is lily, lord of memes and bad builds. I'm sure no one really cares who i am though so lets continue. You wanna play ADC Thresh and one shot everyone, well i'm your guy.
this build is very reliant on you winning lane, despite your'e weak laning phase, hence why its not meta, but it can still work. Its easier if you have a jungler willing to gank bot often, but it can be done without. The key is to play safe and let the enemy just push you in, this incentivises ganks and gives you a chance to pull them under tower, for easy early kills, or free farm if they respect your hook. Just keep up in farm and you will out scale the enemy, be patient.

So this is the bread and butter of your oneshot. Your rune choice makes or breaks the build. Dark harvest is a must, its a large part of why your one auto can nuke squishys. Be sure to abuse free procs in lane after catty minions die. Up next is sudden impact, your hooks dash procs this and it really helps, this is key in the midgame, land hooks on isolated squishys, dash, and pick them off with a big slap. After that is relentless, honestly this one can be swapped out for the others, i just like the ms for roaming, so pick what you like. Next is triumph and last stand, pretty self explanatory, just more dmg and gold, alternatively you can go sorcery and get gathering storm plus nullifying orb to help you lane and for free ad.

Pros

Huge first hit
solid sustained dmg
fairly good cc for staling in duels

Cons

very squishy
low range
can be fooled by dodge/blind
tanks walk all over you

Early game: early game you are a *****, you fear everything, just try to farm and capitalize when your support does work, if you can get early kills with a hook + your supports dmg, go for it.

Mid game: this is where how well you did early counts, if your rocking some good items, solid stacks, and some sweet sunglasses, you can do some roaming. Don't be afraid to run mid and flash nuke a half health mage, just make sure you push lane first. Also at this point you can really poke your laner, anytime they try to farm hive them a slap, it'll scare them off until the jungler ganks, but if your roaming he cant get you.

End game: you have ascended past super sayain god, and now your ready to roll. You grab a Dark Harvest stack, you hook a squishy, and you start one shoting, at this point you can nuke most non tanks, and anything that lives just hook, flay, auto and its dead.