I have an idea, I am playing a classic contract-type fiend warlock very much a "sign on the dotted line and you will have ____.... in exchange for your soul" and I was planning on using Illusory Script for contract fraud and want to know if it would work. For example, I write up a contract that says "I (warlock name) will give you a cup of tea, in exchange, you (victim name) will give me your soul... sign here _____" But then use an illusion to make it bog-standard fair contract. Obviously, the areas you sign are the same on both versions. Are there any obvious issues or workaround I would worry about?
It is up to the DM to determine how cosmic powers adjudicate fraudulent contracts.
Mechanically, this works out just fine. Illusory script says:
You write on parchment, paper, or some other suitable writing material and imbue it with a potent illusion that lasts for the duration.
To you and any creatures you designate when you cast the spell, the writing appears normal, written in your hand, and conveys whatever meaning you intended when you wrote the text. To all others, the writing appears as if it were written in an unknown or magical script that is unintelligible. Alternatively, you can cause the writing to appear to be an entirely different message, written in a different hand and language, though the language must be one you know.
On the player facing side, this will appear to work as you have described. The actual contract will be written however you like, and the spell will make it appear however you like.
But how cosmic third parties relate to this fraudulent contract is entirely up to the DM. Only your DM can tell you if the contract will be binding at all, and if so, which form of the contract will be binding.