Here's why:
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Thresh has awkward mixed scalings. AP is bad on him because a lot of damage he does is delayed so he can't exactly burst and while full crit AD "electric death thresh" build is friggin hilarious (Infinity Edge, Statikk Shiv, Trinity Force, Last Whisper, Bloodthirster, Youmuus Ghostblade for reference) and with it you can often just hook+autoattack someone for all their HP, I don't tend to build that unless we're up like, eight kills a tower and a dragon 10 minutes in. So, damage builds don't work great.
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Thresh's Q flay and ult all want him right in the middle of the action. Ranged autos aside, he's honestly closer to Leona than Blitzcrank, he can't pull people THAT far out of position. The hook is pretty much a stun+gapcloser.
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He's pretty darn mana-efficient for a support. He's not a Sona, or Soraka who have to keep spamming one move for poke damage to stay relevant-thresh waits for an opportunity, then rock'n'rolls. You can win fights with one lantern and a well placed QER combo if you're smart, while Soraka would need to QQQ oh no ally's hurt RWWWWWW oh no someone's diving EQ-you get the idea.
My general purpose Thresh build is this:
Start: Relic Shield, ward, red potion. Yes Relic Shield. When you go up to harass the enemy, autoattack them, then hit flay to kill a couple low HP minions. Or just use your flay bonus to smack a minion.
First Priority: Sightstone. Second Priority: Targons Brace and a ward or two (if you can't afford sight. Third priority: just wards and pots. Do not buy boots first back.
Build (in order of items you should complete after the priority buys are all gotten:
- boots. He doesn't need em much but he does need em. This is the "of speed" variety
- Sunfire (if winning), or Frozen Heart (if losing to enemy ADC) Thresh is great with Sunfire as it's super easy for him to stay in the thick of it with his slows but use FH if losing as you'll want the mana-thresh can't be so aggressive if behind so you'll be doing more flaying and lamping.
- Spectres Cowl later on into Banshees Veil. It's a great buy when you want to not eat shit and die from enemy burst casters or AOEs. Upgrade it to banshees veil when you're late in the game enough when you get deleted when you engage.
- Ruby Sightstone
- Boots of Mobility or Mercuries. Mercs if there's some cc like a Veigar wall you just can't avoid. Or lots of AOE magic shit.
- Face of the Mountain. Upgrade it last-not a lot of stats.