Camille (1G) - Ravager

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Camille is a Ravager who gains omnivamp, deals bonus damage, double against units below a health threshold. With her ability Defensive Sweep at only 25 mana, she slices a target for physical damage and gains a shield for a number of seconds. This means she will want to cast repeatedly while acting as a drain tank. She will likely be a strong early game unit, as shielding over and over again is more likely when fewer units are able to target her.
She is also able to pair well with Murkwolf, who is a 2 gold Ravager for a strong early game.
Caitlyn (2G) - Hunter

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Caitlyn is a Hunter with the ability Headshot, which is based on attacks rather than mana. For the Headshot passive, every third attack, her bullet is instead replaced with a Headshot, which deals additional physical damage.
Hunters gain attack damage, and when they focus on a single target, they get additional damage amp. This trait has 2, 3, 4, and 5-piece webs, so it is easy to add additional hunters as ways to kill units for Essence. Look for Cinderling (1G), Tristana (3G), Sivir (4G), and Ashe (5G) to pair with Caitlyn in order to take down some units while also gathering losses as well. Potentially look to keep Caitlyn to pair with Ashe on post cashout boards as well.
Elise (2G) - Vanguard

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Elise’s ability, Spider Queen, allows her to transform into a spider and gain max health. Her attacks while in Spider Form deal bonus magic damage and heal. Subsequent casts grant decaying attack speed for a number of seconds.
She has the Vanguard trait, which makes her a front-row, melee tank. Vanguard pairs with Rakan (1G), Diana (3G), Hecarim (3G), Ancient Sentinel (4G), and Taric (5G). With the ability to pair with a Vanguard at any cost in the game, with the exception of 2 gold, it is very easy to slot in Vanguards for Elise.
Cassiopeia (3G) - Spellweaver

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Cassiopeia has the ability Noxus Blast, where she poisons the target and the nearest non-poisoned enemy, dealing magic damage over multiple seconds. Poisons also stack. This is similar to some poison or “dot” champions we have seen in the past, where the damage over time will clear everything, including sometimes picking off the backline.
Her ability only costs 30 mana, so frequent casts are important! A strong front line plus an additional Spelleweaver, such as Karma, in the early game, and she will take out a lot of units. It is also possible that Cassiopeia finds herself as a rerollable 3 cost unit, especially with Wisps, which makes rerolling stronger in Set 18 than in other sets.
Morgana (4G) - Invoker

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Morgana is an Invoker, which means she gains additional mana regeneration at 2, 3, 4, and 5 piece traits. Morgana’s ability, Withering Curse, has a passive where it gains omnivamp. With the active, Morgana fires a dark blast at nearby enemies, dealing magic damage to them and cursing them for seconds. Then, she spawns a Hex Withering Zone for the same duration that deals magic damage per second. The cursed enemies take more damage per curse.
Lux (5G) - Avatar

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Lux is a unique 5 cost because the Avatar trait allows her to have different abilities to her ability: Final Spark. For example, the active and passive are the same each time. The passive is on cast, all allies that share a trait with Lux gain mana. If there is something that a lot of TFT players know is that gaining mana in the late game is super strong, as earlier casts can win teamfights and cause your opponent’s units sometimes to die before they are able to cast. She fires her laser towards the largest group of enemies and deals magic damage, reduced by each enemy hit.
There is then a bonus for which trait she has. For example, the Inferno Bonus is to restore mana per takedown. Coven reduces the armour and magic resistance of all targets hit for the rest of the combat. There are other traits that all have their own abilities as well. Lux will be a premium part of late-game boards and will likely be a unit that holds spatulas very well, as the passive makes those units gain mana.
I would not be surprised if an upgraded late-game board with Lux, Ahri, and Karma as the 3-piece Blossom bonus for Wisps sounds strong, and Lux and Ahri sound like a good combination.
Thanks for reading! If you want to try creating your own team comps for Set 18, check out our Team Builder. For a full overview of the new set, check out our TFT Set 18 Reveal Page.