Introduction
Hi, my name is ShoDesu AKA J4NNA. I am a challenger support main that plays on the NA server and play Senna a decent amount on my main. If you have any questions that his guide does not cover, feel free to come to my stream and ask them! My twitch is https://twitch.tv/shodesu

Pros

+ Infinite Scale + Can solo carry + Counters most Enchanters + Global Ult + Unique support

Senna counters all enchanters and infintelty scales. She has a lot of presence on the map as her ult is global and she is one of the most unique supports as she can solo carry games.

Cons

- Hard to blind pick - Struggles against hook champs - No strong peeling abilities - Super Squishy

Senna does not fit into all team comps and has a lot of hard counters making her not blind pickable. You have to position very carefully as she does not have good peel abilities like most traditional enchanters.

Passive - Absolution When units die near Senna, their souls are periodically trapped by the Black Mist. Senna can attack these souls to free them, absorbing the Mist that held them in death. Mist fuels her Relic Cannon's power with increased Attack Damage, Attack Range, and Critical Strike Chance. Attacks from Senna's Relic Cannon take longer to fire, deal bonus damage, and briefly grant her a portion of her target's Move Speed.

Q - PIERCING DARKNESS From the twin barrels of her Relic Cannon, Senna fires a unified beam of light and shadow through a target, healing allies and damaging enemies.

W - LAST EMBRACE Senna sends forth a wave of Black Mist. If it hits an enemy it latches onto them hungrily, rooting them and everything nearby after a brief delay.

E - CURSE OF THE BLACK MIST Senna draws the Mist she has stored in her weapon into a storm around her, embracing darkness and becoming a wraith within. Allies who enter the area are camouflaged and also appear as wraiths as the Mist shrouds them. Wraiths gain increased Move Speed, are unselectable, and hide their identities.

R - DAWNING SHADOW Senna calls upon the relic stones of fallen Sentinels, splitting her relic cannon into a holy array of shadow and light. She then fires a global beam that shields allies from harm, while damaging enemies caught in the center.

FLEET FOOTWORK

VS

SUMMON AERY

Fleetfootwork provides Senna with more movement speed which I believe to be more valuable.

Aery provides more consitent damage/shields. I think the biggest trade off in both of these keystones are the runes in each tree. I believe the precision tree has a lot more runes that fit better with Senna's kit.

FLEET FOOTWORKI prefer to take Fleet Footwork because of the movement speed boost. Senna doesn't have a lot of GOOD peeling tools for when someone jumps on you, W is nice but it takes 1 second for it to trigger, E is nice but if an enemy is in your reach you will show.

Presence of Mind PoM is great for Senna because she has mana issues.

Legend: Alacrity Alacarity give attack speed which is nice for our lethality build, as most of the items dont give ANY attack speed.

Cut Down Cut down is great because Senna's base HP is super low and we dont build any HP items, so most champions will have more max health than you.

Biscuit Delivery Biscuits are nice to help with Senna's mana problems early.

Approach Velocity Approach velocity works really well Piercing Darkness. You are able to automatically follow up on any play after it because of this rune.

I will list out the tips and tricks on this section, but I go over in detail about them in my video which is less than 5 minutes.
Tip #1) Make sure to trade with auto q auto, rather than q auto auto. (you save time by animation canceling.
Tip #2) Look for Q snipes in lane. (use minions/wards to look for angles)
Tip #3) Make sure to be prepared for when souls drop out in laning phase. Grab them BEFORE they go too far.
Tip #4) In post-laning phase, make sure to follow junglers around their camps while pushing out waves to maximize farming and ganking potential.

Senna is a fantastic champion that has a lot of skill expression. Even after her nerfs in 12.12 she is still a great pick into certain team comps. If you enjoyed my guide, please check out my twitch at Twitch.tv/shodesu and my youtube channel at https://www.youtube.com/ShoDesu Thank you!