Notes

Another thing though
The cube isn't alive.
It's an object being controlled somewhere, or controlling itself. But it shows no sign of life, only actions.
If you look at this, it makes me think it's using power to put itself into a real form. The shadow cubes give an effect that makes the player look similar to this, which obviously came from the cube. Lightning is generated by extremely high levels of power, but the cube showed no signs of using energy when printing the runes. It didn't have any degrading to show usage of energy, or any other form of visibly showing a lack of "remaining energy" - So it's either one very powerful boi, or it generates its own power, or potentially draws it from something else - In this case, now seemingly to be the runes
https://cdn.discordapp.com/attachments/483592427513577482/496418897524949002/unknown.png
Not just that, it must have gained a tonne of power to be able to liquidate, dig down under the island. force a huge chunk of land upwards, keep it there, move it all while also creating a storm around it. The runes were most likely generators or power harvesting devices.
One thing people don't focus on enough is the change in state. Solid to liquid to solid.
Or, solid to dissolving into the lake, then back to the solid. Biologically, if you blend a sea sponge, then put it back into the ocean, it reforms in the exact same shape as it was in before. The cube was clearly meant to go to loot lake, otherwise if water was effecting it, it wouldn't have crossed rivers. It purposefully dissolved into loot lake. Then, to reform in the same shape and size, with the same properties and appearance, the "melting" or "dissolving" cannot have been an accident.
Not even its creation, it's design. It couldn't have just formed out of nowhere with no living conscious being forming it and have the algorithms already pre-set to print runes and follow the path. Somebody has sent the cube as a weapon to attack, cause chaos, and if my theory about the dark purple ghost-like object is correct, potentially use the cube to create life. The shadow cubes left at the rune locations weren't any accident. I don't really think this would be correct but the shadow cubes may have been left there by the cube as a way for the cube to experiment with turning the humans of the fortnite world into something similar to what it is now creating. Although, it could be wrong.
The cube went directly from loot lake to the rune location. No delays, no odd paths. It learnt the best way to move around, which is clearly via the air. Potentially (but highly unlikely) linked to it learning the battle bus and humans gliding was efficient. On its original journey it rolled around inefficiently, potentially recharging energy between rolls or maybe even waiting for instructions from whoever sent it. It went in indirect paths and wasn't fast. This time it's going in direct lines as mentioned, between the runes. Could just mean that it learned a better way of transportation, or maybe that it was searching for the places to print the runes in the initial journey, and is now using those runes as pinpoints to return to.