Riot Games finally revealed the upcoming support/marksman hybrid's abilities and they pretty much reflect mechanics seen in Thresh and Lucian, which is fitting, given the characters' history.
Following SKT T1's beatdown of Splyce, Riot Games cheered the fans up by revealing Senna, the Redeemer. A few days later, on 29 October, a video with ability names and descriptions finally surfaced. Considering she is Lucian's wife whose soul got trapped by Thresh, it's only fitting that her passive borrows from the Chain Warden and her Q from the Purifier.
Absolution (Passive) - Senna's cannon is slow to shoot but deals bonus damage. The damage, range and critical hit chance can be increased by collecting Mist from enemies she hits, or from wraiths of dead enemies, much like Damnation increases Thresh's armour and ability power on soul pickups.
Piercing Darkness (Q) - Fires a beam of darkness mixed with light with the impact range and width being fairly similar to Lucian's Piercing Light. Enemies caught in the beam get damaged while allies in the beam, including Senna, get healed. Auto attacks reduce Piercing Darkness' cooldown.
Last Embrace (W) - Sends Mist in skill shot manner. If it hits a champion, they will get damaged and rooted along with any nearby allies following a brief delay. Hitting a minion can trigger the same effect, while last-hitting said minion will trigger the effect immediately.
Curse of the Black Mist (E) - Puts Senna into mist form, granting her and nearby allies bonus movement speed and camouflage. Allies that leave the mist will appear as non-targetable wraiths until they attack or get too close to the enemy champions.
Dawning Shadow (R) - Fires a global, lane wide beam. Enemies in the centre of the beam get damaged while allies in any part of the beam get shielded.
Considering that ulti is basically a global Lux laser combined with Locket of the Iron Solari, it's bound to cause some controversy with the players. In case that's not controversial enough, the group camouflage might just be.