Senna
Living is the Cure
Lane harassment and consistent damage output.
Sorcery
Empowered abilities and resource manipulation
Summon AeryYour attacks and abilities send Aery to a target, damaging enemies or shielding allies.
Manaflow BandHitting an enemy champion with an ability permanently increases your maximum mana by 25, up to 250 mana.
After reaching 250 bonus mana, restore 1% of your missing mana every 5 seconds.
Absolute FocusWhile above 70% health, gain extra
adaptive damage .
Gathering StormGain increasing amounts of AD or AP,
adaptive over the course of the game.
Precision
Improved attacks and sustained damage
Legend: BloodlineTakedowns on enemies grant permanent Life Steal, up to a cap. Once the cap is reached, increase your max health. Weaker earlier but stronger later game than other Legend Runes.
Cut DownDeal more damage to high health enemy champions.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Summon Aery
Senna ability to deal repeated offensive damage with auto's and defensive heals with Q allow you to make excellent use of Aery
Why Manaflow Band
Manaflow allows you to sustain your mana pool in lane with the guaranteed Q hit.
Why Absolute Focus
It's fairly easy to remain high health as Senna thanks to your Q's healing, allowing you to make great offensive use of Absolute Focus, especially in lane.
Why Gathering Storm
Gather Storm is your lane game insurance policy.
WhyLegend: Bloodline
Bloodline allows you to sustain thanks to your high base damage.
WhyCut Down
Senna has very low base HP so you'll be making constant use of Cut Down.