Having made his decision, Isaac immediately set off for Lake Town. He wanted to get it done quickly; afterward, he had to use the White Whale's Infinite Evolution. After days of hesitation, he had decided to give it to Dragon Lizard No. 1.
Giant Dragons are High-level Demons, stronger than other Demons, if anything. Even if it wasn't used to devour the power of other Outer Gods, just using it to purify Dragon Blood would be more than worth it.
Dragon Lizard No. 1 was the first Dragon Lizard to awaken its Dragon Blood with his help. It had also taken Isaac's 'first time,' even before Shushu. As such, it held a special status, just like Shushu, and wasn't sold off like the rest of its kind.
Unfortunately, the Dragon Blood Purification experiments were shelved for a time due to various reasons, and so it was forgotten by Isaac. Now, as Isaac restarted the project, he remembered it once again.
