Actually, it's more or less implied Hagoromo did unlock the third eye as he has a rinnegan seal or possibly even a tomoeless third eye on his forehead.
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The colored version being red only further supports that. Of course it could also be a tattoo, but I believe it's too large and colorful to simply be a coincidence.
Bingo. In part, this is what I believe and here is the proof.
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Two moons, not one. Of course I'm sure people will debate that is the sun, but clearly it is night in those panels...
Here is where my theory comes in. The place Kaguya came from based on the original story about her called the Tale of the Bamboo Cutter is the moon. There, a civilization of moon people dwelt, came to Earth and took her back with them. They are often depicted as either rabbit people or people with large ears or horns as she possesses. Hagoromo also stated the people referred to her as the Rabbit Goddess. Perhaps there was already a moon before Hagoromo created the present day one and it was destroyed somehow, possibly by Hagoromo himself in order to break Infinite Tsukuyomi, but he needed another one in order to seal the Gedo Mazo to keep it out of the wrong hands, as well as possibly to keep the tides from flooding the lands and the ecosystems from being destroyed lols. The sword of Nunoboko was said to have been used by Hagoromo to shape the land. Perhaps this was needed after his battle with either Kaguya or his brother or the destruction of the first moon. With this in mind, I think Hagoromo may have told Sasuke the jutsu to destroy the moon off panel. Possibly summoning the full force of the deva path or maybe even all paths.
