While I am not sure if such a crossover is necessary considering that the MMORPG is based within a fantasy world unlike our own, En Masse Entertainment have recently partnered with Sanrio to bring Hello Kitty to the realm of TERA Online.
From now until Thursday May 19, players are able to purchase a number of different Hello Kitty themed items from the TERA Store. These items include pet mounts, accessories and a number of costumes which only seem to be available as random drops through Loot Boxes. While the Hello Kitty Adventure Loot Box does provide a guaranteed random drop of one of six random accessories – the costume boxes only guarantee the drop of a Semi-Enigmatic Scroll.
According to the store pages, once the crossover sale ends on May 19th at 10am PDT, “these items will leave the TERA store forever”.
You can see all these items listed on the TERA Store.