Context and Overview
If you've never played Teamfight Tactics (I'll refer to it as TFT for shorthand) before, don't worry. The paragraph below should give you a rough idea of what a set is.
Essentially, a set in TFT maps out the framework for the game. For example, in chess, you have certain pieces (knight, rook, bishop etc.) that move certain ways (in an L, vertical and horizontal, diagonal etc.). If I were to change the pieces and the way they move, I'd have a new "set" for chess. For example, if I replaced the pawn with a piece called the jester, which can move diagonally and in an L, I've added a new piece, but the basic fundamentals of chess are still present, like checks, promotions etc. The same logic applies for TFT, this set concept includes new champions with new abilities and traits in order to create a new gameplay experience, while still keeping the fundamental concepts of TFT.
I'll be using this whole chess analogy throughout this showcase to hopefully provide more context about all the different aspects!
Champions - 1 to 4 costs
In general:
1 costs are similar to pawns
2 and 3 costs are similar to minor pieces
4 costs are similar to rooks and occassionally queens




Champions - 5 costs
similar to rooks/queens in chess, since they're generally the strongest units, they'll get a more detailed explanation

Ability: Flawless Duet
Irelia sends a blade to the closest and farthest enemies within 3 hexes. Once both blades have been placed, they fly toward each other and converge, afterwards dealing magic damage to all enemies within a line between them, stunning them and marking them as Unsteady.
Special Trait: Surge
Every 2 seconds, Irelia dashes towards the nearest enemy, dealing an additional 10% of their health as magic damage. If the enemy is unseady, deal true damage instead, dash again, and gain 5 mana.

Ability: Unstoppable Onslaught
Sion charges towards the farthest enemy, knocking aside all other enemies and dealing damage. At the end of his charge, he leaps forward, slamming the ground beneath him, dealing magic damage and stunning impacted enemies.

Ability: Wingborne Storm
Udyr summons a blizzard around himself for 4 seconds that deals magic damage every 0.5 seconds to nearby enemies and slows their movement and attack speed.

Ability: Fear Beyond Death
Urgot fires a chem-drill at the lowest percent health enemy, dealing physical damage and leashing them for 4 seconds. If the enemy’s health lowers past %, Urgot pulls them in, executing them and gaining 40 mana. Enemies near Urgot are stunned with fear after an execution.

Ability: Dark Procession
Yorick summons a ring of spirits for 5 seconds at the center of the board, pulling in the closest ally and enemy and knocking aside all other allies and enemies. Units within the ring deal and take increased damage. While the ring is active, Yorick stops attacking and heals allies for a pecentage of all damage done in the ring.
Special Trait: Gravedigger
After 8 seconds, if at least two allies have died, summon the madien of the mist. The Madien cannot die while inside Yorick’s ring.

Ability: Satchel Charge
Ziggs hurls charges to two random enemies, remaining there for 1 second. Afterwards, the charges explode to deal magic damage to nearby enemies and knock them away from the center of the explosion.
Classes and Genera
similar to the different possible moves in chess
Keeping the whole wilderness/biology thing going, the two trait sections are listed as Classes and Genera. Now I know that doesn't really make sense from a biological perspective due to how traits are assigned, but bear with me here. (yes that bear joke was intentional [ly bad])
Afterlife units gain ability power, armor, and magic resist. Each death during combat increases this bonus individually. When an Afterlife ally dies, it blesses all remaining Afterlife allies with its bonus stats, split equally among remaining allies.
(2) +15 resistances and AP; additional +1 per death
(4) +25 resistances and AP; additional +2 per death
(6) +35 resistances and AP; additional +3 per death
(8) +45 resistances and AP; additional +5 per death

When Basilisks are isolated in a line, they gain bonus movement speed and attack speed.
(3) +10% movement speed and 10% attack speed
(5) +20% movement speed and 20% attack speed
(7) +35% movement speed and 35% attack speed
(9) +50% movement speed and 50% attack speed

Beasts gain additional Attack Damage and Ability Power. Use the Bonetooth Necklace Item to choose a main hunter. The main hunter gains double stats.
(3) +10 AD, +15 AP
(5) +20 AD, +30 AP

Summon a mine cart at the end of your bench. Each round, a moveable crystal spawns on your board. If the crystal remains alive at the end of combat, it is added to your mine cart. Crystaline champions gain stats based on the amount of cystals you have.
(3) +1 Ability Power, +15 Health per crystal
(6) +3 Ability Power, +35 Health per crystal
(9) +8 Ability Power, +75 Health per crystal

Deadeyes mark their enemies with their abilities, causing subsequent deadeye attacks or abilities to deal additional magic damage for the duration of the mark.
(2) marked for 2 seconds, 5% bonus damage
(3) marked for 3 seconds, 7.5% bonus damage
(4) marked for 4 seconds, 12.5% bonus damage
(5) marked for 6 seconds, 15% bonus damage
(7) marked indefinitely, 25% bonus damage

Explorers traverse the wilderness and discover loot. At the end of each combat, the journey counter is reduced by one, when the counter hits zero, they drop a bag filled with what they have collected and the counter resets.
(2) 2-combat journey
(4) 3-combat journey
(6) 4-combat journey

Sandshroud units gain bonus ability power when shielded. Bonus ability power applies to shield casts as well.
(2) +20 Ability Power
(4) +40 Ability Power
(6) +60 Ability Power
(8) +100 Ability Power

Sixth Sense units transcend, gaining additional bonuses.
(1) Regenerate 3 mana per second
(2) Additionally gain 30 armor and magic resist
(3) Additionally gain immunity from crowd control for 8 seconds at the start of combat

Stingers apply a stack of poison with every attack. At 10 stacks, enemies are poisoned and take magic damage over 8 seconds. Poison deals:
(2) 5% of unit’s maximum Health
(4) 15% of unit’s maximum Health
(6) 30% of unit’s maximum Health
(8) 45% of unit’s maximum Health

Vertoxins summon an additional ally. This ally gains bonus stats per star level of Vertoxin units.
(3) Summon Ivern, additional 5% stats per star level
(5) Summon Zyra, additional 10% stats per star level

Innate: Gain +2 attack hex range. Artillery units gain bonus attack speed for each hex between themselves and their target.
(2) +2 attack damage
(3) +5 attack damage
(4) +10 attack damage
(5) +15 attack damage

Innate: Before combat starts, Assassins leap to the enemy backline. Assassins' spells can Critical strike critically strike and they gain bonus critical strike damage and bonus critical strike chance.
(2) +15% crit chance and +20% crit damage
(4) +30% crit chance and +40% crit damage
(6) +45% crit chance and +60% crit damage

Brawlers gain additional maximum health and attack damage.
(2) +400 health and 10 attack damage
(4) +700 health and 20 attack damage
(6) +1100 health and 40 attack damage
(8) +1500 health and 90 attack damage

Cutthroats cause enemies to bleed at low health, dealing additional physical damage over 5 seconds and reducing healing by 20%.
(2) At 25% health, 10% max health bleed
(4) At 45% health, 20% max health bleed
(6) At 65% health, 35% max health bleed
(8) At 85% health, 50% max health bleed

Gain mana whenever an ally or enemy casts an ability.
(2) 3 mana
(4) 6 mana
(6) 10 mana

Fortified units gain a shield equivalent to a percentage of their max health at the beginning of combat and at fifty percent health.
(2) 20% at full, 10% at fifty
(4) 35% at full, 15% at fifty
(6) 50% at full, 25% at fifty
(8) 100% at full, 50% at fifty

Mages cast their spells twice and have modified ability power.
(3) 75% of Ability Power
(5) 100% of Ability Power
(7) 125% of Ability Power
(9) 150% of Ability Power

All allies gain bonus magic resistance.
(2) +50 magic resist
(3) +100 magic resist
(4) +175 magic resist
(5) +300 magic resist

Scallywags deal extra damage but take increased damage.
(3) +5% damage done, +5% damage taken
(5) +15% damage done, +12.5% damage taken
(7) +25% damage done, +17.5% damage taken

Slayers gain omnivamp based on their missing health.
(2) 20% omnivamp, 50% missing health
(4) 30% omnivamp, 40% missing health
(6) 40% omnivamp, 30% missing health
(8) 50% omnivamp, 10% missing health

Vilethorns ignore armor and magic resist for their attacks.
(3) 15% of resists
(5) 30% of resists
(7) 50% of resists

Set Mechanic - Something's in the air!
similar to implementing a chess variation, like no-castling, fog of war etc.
This last section might be a little confusing, so I'll provide a bit more context about TFT. Stages in TFT are pretty similar to moves in chess; at the beginning, there's not many moves you can do and the game becomes harder and harder the more moves you make. Now imagine after every 10 moves, one of your pawns automatically becomes a queen, regardless of its position on the board. This is somewhat the idea of a set mechanic, it provides everyone in the game with a buff to make the game more exciting.
At Stage 2-4, 3 out of 8 dragons shown below will appear will appear around the carousel, indicating possible dragon buffs for the game. At 2-7 PvE (krugs), a final dragon will be selected and shown on each player's board, granting its buff to the entire lobby for the rest of the game.

Mountain Dragon
Grants +30 armor and magic resist for all units. Gain a target dummy.

Cloud Dragon
Grants +5% movement speed and +5% attack speed for all units. Your little legend also moves faster.

Chemtech Dragon
Grants +30 ability power and +20 attack damage for all units. Gain one reforger and one completed item.

Hextech Dragon
Grants +100 health and +10 mana for all units. Gain one hextech remover and one completed item.

Camavoran Dragon
Gain one tomb of traits and 3 item components.

Stalking Dragon
Gain 2 Thief’s Gloves.

Empyrean
Gain a Neeko’s Help, a lucky dice, and 10 gold

Elder Dragon
Grant 3 random bonuses from other dragons.