Champion strength as a support comes from his incredible amount of utility, CC, and AoE, stemming from his ability to displace and disable enemies with his Death Sentence, Flay, and The Box. His Dark Passage can be used as a reliable escape or a way for allies to easily follow up with him. His passive, Damnation allows him to continually gain power as the game progresses to make him deceptively strong and durable. However, champions with strong disengages can hinder him from utilizing his power to his full potential.
While Damnation allows Thresh to continually gain power, he starts off weak. Be cautious early game.
Throwing Dark Passage on to nearby souls can let you collect them without getting close.
This tactic can be useful if you are in a lane against champions who can be more aggressive than you earlier, have a fair amount of poking abilities, or ways to counter your CC (e.g. Lulu, Ezreal, Morgana, and Braum.
Avoid using this method too much as Dark Passage has a long cooldown and can leave you without mana to use your other abilities.
At rank 5, with no attack speed bonus, it adds ~24% of your attack damage per attack. Which is ~20.5 at level 18, without any damage bonuses. (Pending Testing).
It's worth saying that the passive effect does not encourage continual attacking, as you will not get any bonus damage if you do not let the ability "charge".
Death Sentence is a great initiation tool. Consider the merits of pulling them vs pulling yourself. Keep in mind that it can also be cast on neutral monsters. This can be used to escape, set up jungle ganks, or to simply increase mobility.
While landing a Death Sentence is often a decent way to engage, it may not always be the most reliable option; simply walking up towards the enemy and using Flay towards yourself can make it substantially easier to land a hook or at least blow an enemy Flash.
Dark Passage is a great way to save an ally. Not only do you shield them, but you can allow them to escape if they click the lantern.
Consider teaming up with melee champions who have dashes, such as Xin Zhao or Jax. They can jump on the enemy, be shielded from counterattacks, and jump back to safety.
If an ally is coming in to gank your lane, you can place the lantern in his path and initiate, the jungler/ally can use the lantern to jump to you and help secure the kill quickly.
Dark Passage can be a good scouting tool, particularly when checking the Baron and Dragon pits. However, be wary of the ability's long cooldown.
Flay is a good escape and chase tool, depending on the cast direction.
If you catch a fleeing enemy with Death Sentence immediately use your Flay to push them further towards your allies or turret, securing their doom.
Just as Flay's active is useful, so is its passive. Wait for it to charge, and then autoattack an enemy champion for decent harass damage. This method is especially useful in the early game.
If an enemy is camping inside The Box and refuses to touch one of the walls, consider using Flay to push them into one. The slow and damage will be worth it especially if you Flay them towards a turret or allies.
With abilities that scale off ability power and attack damage, as well as having heavy crowd control, Thresh can make use of many stats, allowing him adapt to changing situations and match ups, but is most often played as an off-tank support.
Thresh has many starting options as a support depending on how he's played.
To clarify, the execute component from Targon's BraceTargon's Brace only works with melee attacks and Champion are ranged at all times; however, the damage build up from Flay allows him to last-hit much easier than other ranged supports and activate the heal.
Champion ability power ratios are fairly low, but he gains extra on-hit magic damage, making magic penetration from items like Abyssal Scepter a valuable stat.
Locket of the Iron Solari is probably the best item when playing support, thanks to the fact that he can shield the whole team already with Dark Passage, and increase magic resistance of all the teammates.
Champion effectiveness in several roles depends very much on the number of soul he has collected.
Under 100 souls, your armor is lower than that of the most champions. Thus, you should consider buying items with high armor values or play defensive.
Between 100 and 200 souls, you can consider building something more offensive or refocus on magic resist or health if you are building as a tank. You have matched the armor of many champions, and with 115 souls you surpass Maokai, having the highest base armor at level 18 (passive abilities, such as Shatter not included).
Since Thresh receives a lot of his power through Damnation, zoning him out from collecting souls is especially effective. Take into account your own minions' positions as these are where souls may spawn.
Thresh will commonly use Dark Passage to collect souls. It can be a good idea to wait until Dark Passage goes on cooldown before going on the offensive.
Remember that every death leaves Thresh with downtime where he can't collect souls. His passive can be his downfall if successfully utilized against him.
Thresh has high offensive capabilities, but only in a relatively close range.
Death SentenceThresh is his main initiation ability, and much like playing against other aggressive supports, avoiding it will limit his ability to get on top of you and unload his combo.
Using allied minions to shield yourself can be effective, but note that Thresh may still choose to pull himself anyway to get in range for his other spells.
Thresh is not very durable early on, and harassing him will make it risky for him to use his spells effectively. Use these opportunities and his high cooldowns early on to force fights.
Be aware that Dark Passage can be used to quickly bring one of Thresh's allies into a fight. Try to gain vision of areas behind Thresh from where his reinforcements could approach.
Dark Passage when used to help his allies escape can be halted or prevented by placing obstacles (such as wards or pets) or moving onto the lantern and blocking it with your own hitbox.