What the Rift Lab holds on this champion: the kit as Riot publishes it, the build the lab reviewed, and the matchups it has documented. Anything the dataset does not record is left out rather than filled in.
Riot's own text for each slot, in the order the champion learns them.
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, deals bonus damage , and briefly grant her a portion of her target's Movement Speed.
Piercing Darkness’s cast range matches Senna’s attack range and its cast time is improved with Attack Speed. It can target anything and applies on-hit effects to enemy champions.
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.
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 movement speed, are unselectable, and hide their identities.
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.
The reviewed purchase order and rune page for this champion. Every entry that has a page of its own is linked to it; an item the catalogue holds no page for is printed as a name rather than pointed at a page that does not exist.
Ability text and the in-game title follow Riot's official Wild Rift champion pages. The tier letter is the external snapshot shown on the tier list, and the build is the Nexus reviewed snapshot — neither is a win rate.
In the matchup data documented in Rift Lab, these are the harder matchups for Senna: Blitzcrank (Support), Pyke (Support), Nautilus (Support), Leona (Support), Thresh (Support). This is a documented matchup list, not a win rate: TheNexus does not publish win-rate or pick-rate figures it cannot source.
Riot's ability text for Senna describes a shield in DAWNING SHADOW (ultimate). Plan around those abilities before a trade or a fight. These notes are keyword matches over the ability text only, not a rating of how strong Senna is, so read the full descriptions on this page.
The Nexus-reviewed core build for Senna, in purchase order: Magnetic Blaster then Serpent's Fang then Mortal Reminder then Boots of Dynamism then Galeforce. Runes: Fleet Footwork, Empowered Attack, Chain Assault, Zombie Ward, Bone Plating. This is a reviewed snapshot (patch 7.2b), not a build ranked by win rate.
Senna is a Marksman and Support by Riot's classification and is played in Support. Riot rates the difficulty High. On our tier-list snapshot (patch 7.2b) it shows S tier from its external source, which is a snapshot letter and not a TheNexus win rate.
Every answer here is built from Rift Lab data and Riot's own ability text. Where the data is missing we say so instead of guessing.
