The two games are gearing up for an official crossover. Terraria and Core Keeper will receive free updates in each of the games. Developer Core Keeper announced on Steam that their update will be released on September 26th and will allow players to summon the Slug King from Terraria:
Now we can announce that starting September 26th you will be able to summon one of the most powerful creatures from Terraria. That’s right, say hello to King Slime. However, this giant king slug does not appear by accident. In fact, he’s a bit like Omoroth in the sense that you need to summon him whenever you want to fight. – How to call him? You’ll have to figure it out yourself! The fight against King Slime can be challenging, but definitely worth the time. Rumor has it that for defeating King Slime you will get some pretty interesting loot that your fellow explorers will envy.
Meanwhile, Terraria fans can look forward to the arrival of the friendly farmer spelunker pet from Core Keeper – it’s one of Terraria’s cosmetic accessories and will follow you and provide you with moral support on your adventures. As with the King Slime update, you’ll have to figure out how to get the Caveling to spawn – though the update post teases that it requires another special item.
The crossover event is permanent, so there is no need to rush. But with Terraria 1.4.4 coming soon and Pugstorm’s next major content update, The Desert Biome, there’s a reason to return to both games.