Alex saw the sword come to replace the buried sword. This was an intriguing Sword Art, he thought.
After reaching this point, Alex couldn't use his Sanguin Sword Art anymore as it was already a bit too weak for him to use. Luckily, he had Seven Stars Cut in his repertoire. But it didn't change the fact that he abandoned one of his Sword Art.
He was wondering whether to replace it with another Sword Art or not. But after he found this sword art, he felt this sword art was perfect for replacing his Sanguin Sword Art.
This Sword Art was similar to his Desolation, where he summoned five illusory swords. But this Sword Art used real swords, and this made it even stronger than Desolation. Another reason why he felt this sword art was a good replacement was because he had too many swords in his inventory.
He never had any chance to use it or sell it. Rather than sitting in his inventory for a long time and being forgotten, he chose to use it with this Sword Art.