This build is the most fun I had with playing sett.
I am not responsible for any lost LP even though it's really effective and helps me climb.
Use it only when you know how to play Jungle and comfortable with Sett

• You may miss being the great duelist (you are still very good but not the best) but you will be very effective on team fights. Your ult and W can destroy the enemy team's health and a few of them even.
• If you're about to get hard CCd (stun or suppress) and you think you probably won't survive to W after that then use it even though it's not at its full power
• You as sett have good early game but fall a bit mid and do great late when getting items.
• You are a team fight focus, make sure your team follows you and try to W their carry. taking him out will make sure your team wins with they low health left.
• You may not be very tanky but you will deal massive amounts of damage.

• Sett has one of the least healthy clears so you always need to be careful of invades early.
• Do not be afraid to use W on jungle camps when you're sure you're not being invaded, it makes your clear much healthier and faster.
• When having some health you can 1v1 any champion early so when expecting a fight use the smite
• Do not use the smite when low health because you're on low health, Sett's passive regenerates health depending on missing health
• You're not the best ganker but you are the best team fighter so farm farm farm
• For sett jungler, your main healing source is [conqueror], stack it as soon as you can and don't be afraid of extended fights.

Main tree
Conqueror Massive healing with the great amount of damage you'll be dealing making you a able to survive duels especially early making you less likely to get invaded and better chances surviving it.
Grasp of the Undying for sett support stacking even more health and healing off it

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Triumph best one because it will help you survive for maybe hitting another W in the team fight
Font of Life any other can also be good but as support healing your adc is always good.

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Legend: Alacrity is my go to since the attack speed helps you stack Conqueror and deal even more damage
Legend: Tenacity is good when facing a lot of CC
Bone Plating as support your early game is very important and surviving enemy burst is crucial for winning the 2v2

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Last Stand best one since you'll want to be dealing most damage when low on health either when W maxed or Conqueror fully stacked
Overgrowth because more max health = more W damage

Secondary tree
Eyeball Collection because W also scales with AD
Conditioning because you don't want to only survive initial burst you also want to survive long enough to throw a second W. That's why I think it's better than Bone Plating
Hextech Flashtraption because engage sup

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Ravenous Hunter gives you good amount of healing, since you'll be dealing massive amounts of damage it has the best value
Overgrowth as explained before
Future's Market because items are really useful
Unflinching vs teams with a lot of CC

Tiamat- should get as soon as you can, boosts clear speed significantly.
Titanic Hydra- first item to complete since it's damage boost is insane and helps you duel and clear camps much faster.
Berserker's Greaves- you get a lot of damage output so the 35% attack speed may seem useless but it helps you stack Conqueror faster which is crucial for your dueling ability and the boost to the damage output from Titanic Hydra is so good. Mercury's Treads only really useful vs CC
Goredrinker- Is crucial first because it has both health and AD. Second, it increases your AD on low health which boosts your W damage (W scales with both AD and health). Third and most importantly its active healing is crucial for surviveability when you ult on their entire team it can save your life, Iceborn Gauntlet is good when your team is in dire need of a tank.
Sterak's Gage- Health and damage for W scaling, and it's shields for surviveablilty
Black Cleaver- Mainly for health and damage for W scaling, but its armor shredding can be useful too. Abyssal Mask also good when facing AP and it's 15% damage increase on enemies can be real good if you have a proper teammates. Thornmail buy only when you REALLY need armor.
Warmog's Armor- is a must for me at least since it's 800 health is hard to pass and leaving the first teamfight for a bit then coming back in full health for the second is hard to pass