Riot Games said that HBO contacted the studio with the offer, saying that there was a shared interest between the two franchise's fanbases. Riot released League of Legends assets to HBO, which will then be turned into a trailer spotlighting both the MOBA and TV series.
The game studio gave an overview of the promotion in an interview today: "HBO is doing a three-week activation in advance of their series premiere on April 6th. They're going to be getting some marketing assets, whether that's logo bugs or website takeovers. But we're also working with their team to produce a mash-up trailer. They're giving us some exclusive sneak peaks into footage that hasn't been seen before."