Am I allowed to determine tenets of my contract as a warlock?

A friend of mine invited me to a short campaign. I chose to play as a Hexblade warlock since I thought that it would be interesting to have a sentient weapon as a patron. As part of my character I thought that it would be interesting if I owed my patron a certain amount of dead that scaled with my level each month as per my contract.

When we were doing introductions in the party, I explained this part of my contract. My DM asked me to roll Deception, stating that rules didn’t say anything regarding contracts, so I didn’t have such an obligation and as such I was lying to the group.

Was he right to shoot me down like that, or is it within my rights to customize my contract however I want?