The League of Legends developers are testing (and will most likely do so) a comprehensive stat overhaul for Senna in League Patch 14.16 to help the support suffer fewer losses—a drop that is largely driven by players continuing to purchase lethal items for the Redeemer.
Goodbye lethality! – Welcome AP? (Senna changes to PBE)
After these planned Riot changes, which landed on League PBE this week, Senna should play much more like an Enchanter again. Right now, she's mostly played with a Bloodsong and Edge of Night core.
Senna's lethality has been good in the past, but has now dropped to a meagre 48.34 percent win percentage in season 14, according to statistics website U.GG.
To address Senna’s issues in the bot lane, Riot is making changes to her base stats and her passive and Q, W, and ultimate abilities. These range from a lower attack speed growth through her stats to more ability power and attack damage scaling on her heals, slows, and shields. For her W, her root has been made more powerful to encourage casting. Only her E has been left as is for now.