Original Post — Direct link

Senna's scaling:

Range -

at 20 stacks (approx. 4 min into the game) - 625 range - second highest in the game (only behind caitlyn)

at 40 stacks (approx. 10 min into the game) - 650 range - highest auto attack range currently in the game (tied with Cait)

at 80 stacks (approx 20 min into the game) - 700 range - same as Kindred with 20 marks!

at 160 stacks (35+ min into the game) - 800 range - HIGHER THAN TOWERS ATTACK RANGE! (you can DPS towers outside of their attack range!)

similar to baron buffed cannon minions!

Crit strike -

at 60 stacks - 45% crit strike chance WITHOUT any crit items!

at 140 stacks - 100% crit strike chance WITHOUT any crit items!

Life steal -

crit strike chance above 100% is converted to life steal!

with Bloodthirster and Dead's Dance - you can get well over 60% life steal in late game.

meaning that 1 Q basically refills your HP... (quite similar to current Aatrox when he presses R)

at 200 stacks - Senna can get up to 75% life steal while maintaining 100% crit strike chance and 200 bonus AD!

Q range

scales with auto attack range.

at 80 stacks - 700 range to target + 1300 range of healing beam

CAN HEAL ALLIES AND DAMAGE ENEMIES 2000+ units AWAY FROM HER!

Runaan's Hurricane -

can harvest stacks from multiple champions hit by Runaan's bolts!

Hail of Blades

Senna needs two autos to harvest a stack!

With Hail of Blades she can harvest stacks almost instantly while staying out of range of her opponents

She also deals % HP damage when harvesting a stack - meaning that armor stacking is not very effective against her

Slow downs

- harvesting a stack slows down the enemy! meaning it's not easy to escape from her once she autos you the second time!

(similar to how Tristana's jump slows you down)

Movement speed

- her E gives 20% BONUS MOVEMENT SPEED TO SENNA AND ANY ALLIES IN RANGE! FOR 8 SECONDS!

(basically like the current Shurelya's Reverie - only 5 times better)

INSTANT ROOT FOR 2 SECONDS!

killing a minion with W will immediately root ANYTHING withing radius of 280 units! FOR 2 SECONDS!

External link →
about 5 years ago - /u/Auberaun - Direct link

Just for some context - support Senna usually won't exceed 140 souls in a game (and that's in a very long game where she gets a lot of souls), ADC Senna usually won't exceed 100-120 (again in a long game where she's really successful at getting souls). Games ending anywhere between 60-120 souls depending on role and success in the game is normal.