The moment he decided not to reveal it, he had made an even bigger decision—to never even think about it again. Not because he didn't want to, but because according to Lia and the system itself, even he couldn't control it.
But power had a way of demanding attention.
The stronger he became, the harder Ruin fought against its chains. The system had been doing its best to keep it locked down, but it had reached its limit. Unless Pyris dumped most of his Lust Points into reinforcing the seal, Ruin wasn't staying contained. That's why Pyris didn't even consider ever using these points no matter how much of them he got, somewhere in-between he had forgotten about using them.
That's why he was so relieved that he could use Eternal's Seal without the points because he didn't know how long it will take for him to control Ruin by himself.