Passive:

Damnation:

BASIC ATTACKS: Thresh's basic attacks do not use projectiles and his basic attack wind-up is only reduced by 0.25% per Attack speed icon 1% bonus attack speed (compared to the standard 1% per 1%).

INNATE: Enemies who die near Thresh drop souls for 8 seconds, which he can collect by either approaching them or by placing Dark Passage Dark Passage's lantern near them. Each collected soul permanently grants Thresh Ability power icon 0.75 bonus ability power, Armor icon 0.75 bonus armor and increases Dark Passage Dark Passage's shield's strength and Flay Flay's minimum bonus magic damage by 1.

Champions and large enemies always drop 1 soul, epic monsters always drop 2, but small minions and monsters only have a 33% chance to drop 1. Souls are visible to allies and only become visible to enemies if their team has Sight icon sight of Thresh.

Thresh's passive provides scaling AP and armor based on the number of souls you have collected. The AP helps with ability damage without you having to build any AP items.

P.S: If you are too far to collect souls, you can throw your Dark Passage onto them to collect the souls. However don't do this if you can help it as this ability is used to peel for your ADC.

Q:

Death Sentence:

FIRST CAST - DEATH SENTENCE: After a 0.5-second delay, Thresh throws out his scythe in the target direction and forms a tether with the first enemy hit, dealing Ability power magic damage and Stun icon stunning them for 1.5 seconds. Hitting an enemy reduces Death Sentence's Cooldown reduction icon cooldown by 3 seconds.

While the tether persists, Thresh is Disarm icon disarmed and periodically tugs at his scythe's chain, Airborne icon pulling the target a short distance towards himself. After 0.5 seconds, or instantly upon hitting a minion or monster, Thresh can cast Death Leap Death Leap while the tether holds.

Death Leap
SECOND CAST - DEATH LEAP: Thresh Dash dashes to the tethered enemy, removing the Stun icon stun but becoming able to attack again.

MAGIC DAMAGE:
80 / 120 / 160 / 200 / 240 (+ 50% AP)

THE most important part of Thresh's kit. This is the ability you use to initiate a fight. It is also a gap closer and if used properly, an escape tool. It has a 0.5 second delay, which might alert enemies. Therefore use it properly.

P.S: It can be used to interrupt Teleport channel and can also allow a Yasuo on your team to ult( Last Breath) off of Death Sentence.

W:

Dark Passage:

ACTIVE: Thresh throws his lantern to the target location, Sight icon revealing its immediate surroundings and collecting nearby Damnation souls for the duration. The lantern remains for 6 seconds and automatically returns to Thresh if he moves more than 1500 units away from it. The first ally to come near the lantern is Hybrid resistances icon shielded for 4 seconds.

An ally that grabs the lantern will Dash dash to Thresh's location and shield themselves for the same duration.

SHIELD STRENGTH:
60 / 100 / 140 / 180 / 220 (+ 1 per collected Damnation soul)

One of the most versatile abilities in the game. It can be used offensively or defensively. It can be used as peel for your ADC or any teammate when they are in trouble, or it can be used to pull an ally towards you when you are engaging.

P.S: Dark Passage can be used to provide vision so you don't have to facecheck a bush. It can also be teleported to using Teleport. Spells like Leap Strike and Shunpo can be used on your lantern.

E:

Flay:

PASSIVE: Thresh's basic attacks deal bonus magic damage, increasing with time spent not attacking enemy units or neutral monsters.

ACTIVE: Thresh sweeps his chain in a broad line towards the target direction, dealing magic damage to enemies hit, Airborne icon knocking them in the chain's direction, and Slow icon slowing them afterwards for 1 second.

BONUS MAGIC DAMAGE:
Collected Damnation souls (+ up to 80 / 110 / 140 / 170 / 200% AD)

MAGIC DAMAGE:
65 / 95 / 125 / 155 / 185 (+ 40% AP)

SLOW:
20 / 25 / 30 / 35 / 40%

The second most important ability in your kit. Flay is usually used in sync with Death Sentence. This ability like Dark Passage can be used both offensively and defensively. It can be used to push enemies away or towards your allies. Flay also knocks away enemy jumps( khha'zix's Leap). Using Flay immediately after Death Sentence makes it cast faster.

R:

The Box:

ACTIVE: After a 0.75-second delay, Thresh surrounds himself with 5 spectral walls, each lasting for up to 5 seconds. Walls break upon contact with an enemy champion, dealing magic damage and Slow icon slowing by 99% for 2 seconds.

Enemy champions hitting any wall beyond the first take no damage and are Slow icon slowed for only 1 second.

MAGIC DAMAGE:
250 / 400 / 550 (+ 100% AP

Just as the name sounds, your ultimate is a box which is spawned a short distance around you. Enemies which pass through the wall take huge amounts of damage and are slowed by a LOT. It can be used as an obstacle against advancing enemies or to trap enemies inside long enough for your team to pick them off. Be careful though as it has a huge cooldown time.