Hey Thresh lovers,
this build is all about hard snowballing the game and is a medium risk high reward thing in my eyes because you are still usefull and can support your team even if you dont snowball.
Basically if you hook somebody with 45%cdr and maxed hook he wont escape without using summoner spells and even then they have a hard time. Get those catches with Talisman of Ascension movementspeed and flash hook.

But not only can you snowball the game and do things I wrote above, also you can support your ADC very very well. Just give him Knight's Vow what you rush first so you absorb (if nearby) some of the dmg he would take and you get healed if he deals dmg to other champs. In a teamfight or just with your ADC, Locket of the Iron Solari AND Redemption can win a teamfight or save your ADC because the champs who are affected by those items get a massive shield ( Locket of the Iron Solari gives an AOE shield and Redemption gives an AOE heal) and heal. And last item Talisman of Ascension gives an AOE movementspeed boost to engage/disengage.

Wanderer for extra movementspeed if you roam or run back to the lane its a must have.

Secret Stash for more sustain/mana its a must have as a Support.

Meditation for more mana, its a must have as a Support with Mana.

Dangerous Game is what I prefere because its better on lane if you fight (You want to fight with this build and ignite) or in a teamfight. But you can also take Bandit for that extra little gold in the bank or Greenfather's Gift for extra poke on enemys with your first strong AA out of brush.

Intelligence you go 45%cdr Thresh, what a question.

Thunderlord's Decree gives you more burst dmg, destroy the enemy adc.

Recovery is what I prefere (if you want to, you can also take Unyielding which scales with your passive armor gain through souls) because you get this additional lifereg which scales with Runic Armor what you want to take too.

Tough Skin is always good because you will get poked with AA's from Support and adc but Siegemaster is a great one too against lanes which push you under your tower and want to harass you there.

Runic Armor you take this one because it scales with your Recovery which u took before and of course with your Dark Passage shield, Heal from adc, Talisman of Ascension lifereg, Redemption lifereg, with Secret Stash regeneration, heals/shields from other champs on you in generell.

Insight is a must have on 45%cdr Thresh for more flash/ignite/exhaust you want to hard snowball the game.