"Do not make bargains with Demons and Devils, they can not be reasoned with. Their hunger is absolute."
Tenma remembered these words from his weapon Pride when he first traded for the Scripts to summon Demons and not just any Demon, a Demon Prince, one of the most powerful creatures in all the many universes.
At first, the prospect of being able to summon such an existence was tantalizing, the power he could control, even if it was only for a short while, showed Tenma a glimpse of an authority that even the gods lusted for.
Something eternal, even if it was incredibly wicked and depraved.
He had held onto this summoning for a long time, with the firm knowledge in mind that he would never be using it unless he was at the end of the ropes or the project he was pushing for using both weapons of Sin, then he might need this power.