Two rather great sandbox mining games are set for an official cross-over. Terraria and the new kid on the block Core Keeper will both be getting free updates for it.

Announced by the Core Keeper developer on Steam, their update will land on September 26th and will allow players to summon the King Slime from Terraria:

We can now reveal that, from the 26th of September, you’ll be able to summon a creature you’ll recognise as one of Terraria’s (pre-hardmode) heavy-damage dealers. That’s right, say hello to King Slime. This giant, royal blob doesn’t just spawn randomly, however. In fact, he’s a little bit like Omoroth in that you’ll need to summon him forth when you want to battle. “How do we summon him?” you ask. Well, that’s for us to know, and you to find out! Battling King Slime may be a challenge, but it could definitely be worth your while. Rumour has it that defeating King Slime will score you some pretty interesting loot that will make you the envy of your fellow explorers

Meanwhile, it seems like Terraria will be getting an update on September 28th (source) which sounds like another big one, and seems like the cross-over update will be bundled into it. For the Terraria side it will be this:

Cavelings are usually very hostile creatures, especially towards adventurers. That being said, there are one or two Cavelings that have proven friendly in the past (like the Caveling Merchants). It’s safe to say that Cavelings can be good company, in the right conditions, and it looks like you’re going to be able to make friends with one in Terraria. Introducing the Caveling Pet, a friendly Caveling Farmer that’ll follow your character around looking cute and offering a little moral support wherever you go. You’ll be able to summon this uncharacteristically docile Caveling in Terraria using a special item that you’ll also recognise from Core Keeper. We think it’ll be more fun for you to figure this one out yourselves, so we’ll say no more on the matter!

This is not a timed-event either, with the cross-over update being permanent additions to both games.