Abilities of Senna
Spell skill priority is an important winning factor for Senna. Understanding the abilities of Senna in League of Legends is another. Below you can find all the relevant information on Senna abilities. In order for item builds you should check the build for Senna.
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.
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.
Senna shoots a bolt of piercing shadow through an ally or enemy, dealing totaldamage physical damage to enemies and Slowing them by totalslow for slowduration second(s). Restores totalheal Health to allied champions.
Attacks reduce this Ability's Cooldown by cdreductiononhit second.
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.
Senna sends forth the Black Mist, dealing damage physical damage to the first enemy hit. After a delaytime second delay the target and other nearby enemies are Rooted for rootduration second(s).
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.
Senna dissolves into a cloud of Black Mist for buffduration seconds, becoming a Wraith. Allied champions who enter the Mist are totalms Move Speed, are Unselectable, and hide their identities as long as no enemy champions are nearby.
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.
Senna fires a beam of light that deals totaldamage physical damage to all enemy champions hit. Allied champions hit in a wider area receive totalshield Shield for shieldduration seconds.
FAQ on Senna
What do you max first on Senna?
Most Senna pros max out 
Piercing Darkness first.