Riot Games recently updated the Public Beta Environment (PBE) server with upcoming skins that are supposed to go live in patch 9.17 but it looks like they slapped the splash art of an upcoming champion to one of the skins as a placeholder.
Riot Games seem to have accidentally spilt the beans by messing up the splash arts on PBE, displaying an incorrect one for Elderwood Nocturne. The correct splash art is what you see in the image above and what they put instead can be seen in the picture below.
The picture is of a woman holding what looks like a huge gun with two smaller objects in her left hand. Now, the splash art looks like it's in an unfinished state since the smaller objects look really rough around the edges but it's clear that this is a champion we haven't seen before.
Considering that Riot Games announced an AD carry earlier, it is highly possible this will be her, especially because marksmen tend to have guns or bows.
Fan theories about this mysterious woman are already flaring up, with some believing she is Senna, Lucian's wife whose soul was trapped by Thresh. Both her looks and weapons fit the bill but there is just that teeny tiny detail that stands in the way of this theory - Thresh is holding her soul hostage.
Others are taking a humorous approach. One fan why her smaller weapons look like GPUs, with another one answering that she is obviously an AMD Carry. Whatever the case, Riot Games let the cat out of the bag and we are not talking about Yuumi.