Ruination is coming to Summoner's Rift as data mining revealed assets for an alternate version of the map as well as mentions of a new League of Legends game mode.
Riot Games previously stated that the entire Season 12 will be based on Ruined King and the phenomenon of Ruination that consumes the land and creatures on it. They seem to be ready for the next big themed event as they are adding assets that obviously reference or look the part for the Ruination theme.
Data mining provided the name Ultbook, which may just be a placeholder title for the new game mode, as well as several images of the ruined Summoner's Rift. One of the images is the Baron Nashor under the effects of Ruination and there is also an image of the full map with the new effects, which you can check out below.
It seems like the theme will be Sentinels vs Ruination as the order was mentioned a few times in a few data mined strings. Considering that Riot Games announced a new Sentinel champion that utilises a grappling hook as the next release, it is quite possible this entire event is a buildup to the new addition.
Dr Mundo is also close to release so it's possible we will get some spacing between the two champions. Therefore, the event might either be a teaser for the new Sentinel or the event itself might be coming a bit later.
You can also check out the full batch of data mined information over at Surrender@20 for more screenshots and reference strings.