Let's get this straight.

Traditional way of maxing has been R->E->W->Q. Which is, according to many, a good start.

But in Korea, Poohmandu, previous support of SK Telecom T1 K who retired due to health reason, started maxing Death Sentence over Flay. Also, one of the best Thresh in the NA, BunnyFuFu, maxes Death Sentence over Flay.

Here is how Mandu did it.
  • Level 1: Get Death Sentence or Flay, depending on the Level 1s.
  • Level 2: Get the other one that you missed out.
  • Level 3: Get Dark Passage
  • Level 4~5: Put points on Flay
  • Level 6: Put point on The Box
  • Level 7+: Max Death Sentence

    Here is the reason why.

    Death Sentence at Level 1 has 20 Seconds Cooldown. Death Sentence at Level 5 has 12 Seconds Cooldown.

    Now, max your 40% Cooldown Reduction, and Level 5 Death Sentence has 7.2 Seconds Cooldown. Now, if you grab someone, you get 3 seconds rewarded off from cooldown, so you are remained with 4.2 Seconds Cooldown. At teamfight, this becomes 1.5 Seconds Stun with 4.2 Seconds Cooldown because opponents won't be aware of everything with everything firing up. So basically, there are only 2.7 seconds of Cooldown if you subtract the time of stun duration.

    However, why put 3 Points into Flay before maxing Death Sentence?

    Although we can see a significant Dragon fight happen significantly around Dragon pit, major teamfights during Solo Queue happens after Level 10. Because maxing Death Sentence delays your lane presence, you want to be leveling Flay up until you are Level 5. Put one more Level into Flay, and you have to delay your Death Sentence max until Level 12 because you would get second point on The Box at that point.

    However, if you have significant advantage by end of Level 3 (does not happen often, but it does), you could consider going Death Sentence from Level 4 and onwards because you would be pushing lanes out really quickly, and you want the pick potential from Death Sentence, and make your opponent fall like dominos.

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    Now, the issue after this is: do you max next? Flay or Dark Passage?

    You want to master Dark Passage second.

    Damage in support, unless you builds items to back it up, falls off. Reducing cooldown of a utility, however, does not fall off. Such is this case.

    Thresh's Flay has same cooldown for all the levels. Dark Passage, however, has lower cooldown as you level it up. Although Flay has increasing slow effect the more you level it up, you already have The Box for that. Beside, you use Flay for hard CC purpose mainly. Dark Passage, however, gains enormous usage as you level it up. It can even be a mini-locket for some cases.

    However, situation changes sometimes, such as when the frontrow has no good gap closer - or maybe has one ultimate (such as Shyvanna). But if they are putting Kha'Zix in front of you, (which is happening a lot now) you would rather want Dark Passage.

    So, the Skill Order is: R->Q->W->E.

    Tl;dr