ohhh right, well I was about 8 - 11 years old I think, but it stuck in my memory for a long time because it was just SO odd! Glad that you wrote of it, I've never mentioned it to anyone. I'm 48 now so...it's so long ago. It was just like you said - it comes to you in like a dream, and makes you this offer, but it has such a different quality to it than any dream. I'm 48 now so...it's so long ago.
uhm if you're worried about it, maybe you should look into matters of faith and do a recantation of some type, whether written or spoken, in private or with the assistance of a cleric of some type. I don't feel concerned as I was a child at the time and I have since become a christian, I had a deep conversion experience (from agnostic) a few years ago. Another matter to consider is whether an agreement made in a dreamlike state is binding because, of course, one is not in a 'rational' state. But anyway, it seems like it's worrying you...
Here's a catholic prayer of renunciation, for an example. If you do some googling on the subject you might find something to suit you better.
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Renunciation Of Satan and Claiming the Full Victory
I claim the full victory that my Lord Jesus Christ won on the Cross for me. Having disarmed the powers and authorities, He made a public spectacle of them, triumphing over them by the cross" (Col. 2:15) His victory is my victory.
In the name of the Lord Jesus Christ I renounce all the workings of Satan in my life in all its forms, whether brought into my life by my actions or by others. I break all attachments, ground, curses, spells, and rights Satan may have in my life whether such ground was gained through my actions or through others. Strengthened by the intercession of the Immaculate Virgin Mary, Mother of God, of Blessed Michael the Archangel, of the Blessed Apostles Peter and Paul, and all the Saints and Angels of Heaven, and powerful in the holy authority of the name of the Lord Jesus Christ, I ask you Lord to command Satan and all his minions, whomever they may be, to get out of my life and stay out. With that authority I now take back the ground in my life gained by Satan through my sins. I reclaim this ground and my life for Christ. I now dedicate myself to the Lord Jesus Christ; I belong to Him alone. Amen.
Holy Michael the Archangel, defend us in battle. Be our protection against the wickedness and snares of the devil. May God rebuke him, we humbly pray; and do thou, O prince of the heavenly hosts, by the power of God, thrust into hell Satan and all the other evil spirits who prowl through the world seeking the ruin of souls. Amen
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