Lenny knew that this Devil boss was more interested in feeding on him than all the others. And as such, he presented himself as both the meal, but also the Trap.
Coupled with the fact that he had entered the Semi-Arcane Domain on his own accord, the confidence of the Devil Boss had increased.
And as such, Lenny used this confidence to his advantage.
When Lenny had finished fighting the Second Devil Boss, he had swallowed its core, but had refused to absorb it. All he did was extract the fraction of law from it. The Devil Boss core itself was left to be intact.
Lenny drew a shadow rune on the core, and then he stored it within his body.
The Third Devil Boss had believed that if it took lenny's ability to move, it could feed on Lenny the way it wanted. Also, the only reason Lenny had beaten the other devil Bosses was because he had been able to move.
At least that was what the Devil boss thought.