By and large, demons are not to be trusted.
The promise of a biblical demon is less worse than a sand dollar in Las Vegas. They aren't bound to keep it unless they use their true name when making the promise—and even then, not all demons possess true names to begin with.
Everyone in the heavenly realms knows this. It's why demons from hell are so immensely disliked.
Surely, Percival would know that Ziz, one of God's first creations, would know this.
'So the why is he still talking...?'
"- And then I told him, I'm like, there's no freaking way you met the original Immortal Phoenix- like that's fucking crazy. I mean, I know everyone's got a real hard on for the second one, given that she's got that crazy bod and all, but c'mon. You are the numero uno, man. The blueprint!"
Percival's eyes became giddy like a child's. Ziz was having a hard time believing what he was looking at.