UPDATED FOR S12!

Hello! This is my Seraphine S12 build. She's my favorite champion to play nowadays. Please note- I'm not a pro, but I can flash my M7 (lol).

Seraphine is an incredibly strong early and late game champion with insane CC, supporting, and damage. Her mobility really isn't that bad if you know how to dodge. She's a little broke lmao.

Items:
-Imperial Mandate: Procs on E and R, and Q after imperial mandate. This is so good on Seraphine because E and R are such long range CC abilities.

-Lucidity boots: Just very good for supports right now! You don't really need Sorc shoes because your goal is not damage.

-Rylai's Crystal Sceptor: The way Seraphine's E works is that it slows enemies and roots already slowed enemies, and stunned already rooted enemies. This means that with Rylai's, your E will always root and sometimes stun when her P "Echo" (double cast) is up.

-Chemtech Purifier: This is very good because her W can affect most of her team. This is usually a good buy regardless of enemy team comp, but it can have less priority if the enemy doesn't have much healing.

-Staff of Flowing Water/Ardent Censor: Staff is generally good for every team comp, and Ardent is only really good for attack speed ADCs (Caitlyn, Ashe, Jinx, etc.)

Early Game:

Play aggro against mages/enchanters and after engages are missed for tank/engage sups. Seraphine's Q does very good poke damage. You will be weak to ganks and getting hit early game in general, so winning your lane is very important. Basically, you need to fund your first 2 items quickly (Mandate and Rylai's)because that's when she gets powerful. If you lose lane, you will be pretty useless for the rest of the game.

Since you don't have the biscuits from other Seraphine builds, you will run out of mana quickly. To prevent this, only poke with your Q unless you have amazing aim or an easy target to E. Presence of Mind and Manaflow Band will allow you to stay in lane even with low mana. The mana regen isn't really fast enough to keep up with the amount of poke you need to dish out but it is helpful.

Auto with her P "Notes" when possible- the auto range increases with every ability you or an ally hits. Be aware that your Q will steal some minions, be nice and talk it out with your ADC if it becomes a problem.

Seraphine's P "Echo" allows her to double cast her abilities. This is very important early game and occurs every 3 abilities. Save Echo for your Q if you need to poke, W if you need healing, and E only if you have a very easy shot, such as on a rooted, slowed, or no flash enemy.

Mid Game
Around once you take Tier 1 Bot tower and have Lucidity boots, you should start roaming to help mid lane. Don't forget to take control wards with you and sweeper. However, be very careful of getting picked because your team has a high chance of losing teamfights without you. Always travel safely and stay in a lane with at least one another ally.

Late Game
At this point, you need to be at every teamfight. Use your R even if it only hits one enemy- even this can help your team. Remember, your ult bounces off every champion it hits, both enemies and allies. You can hit an ally first to get a long range engage, but this is risky and it is usually better to hit enemies standing behind each other instead. Besides R, your W and E are your biggest assets. Shielding and antiheal (Chemtech purifier) for the whole team? Yes please. Roots and stuns (Rylai's) for every enemy hit? Yes.

Remember not to get stuck in the teamfight. You are very squishy and will die immedientally if hit by like anything.

I hope this guide was helpful! Have fun on your Seraphine games!