Mutation is a solo writing RPG intended to simulate the creation of those journals one often finds in zombie/post-apoc movies and games, in which a character documents his transformation into a zombie or monster or insect or whatever. Your objective is to write as many entries as you can.
Start by writing your first one. Begin with a date, and describe your situation (where you are, who's with you, etc) and what is going on. Use as many words as you want. Your mutation hasn't started yet at this point.
Before writing each next entry, roll 2d6. As long as both dice are equal or greater than your current entry count + 1, your mutation doesn't start. Keep writing as above. If, however, at least one die is smaller, your mutation has begun.
If you're mutating, before every entry, roll 2d6. Let's call these dice N and M. Look at your previous entry. You can no longer use its N-th word's M-th letter, neither its M-th word's N-th letter. List these somewhere. This represents your mutation eating your mind away.
I.E: you roll 2 and 6. Look at your previous entry. From now on, you can no longer use the second letter from its sixth word in any entry you write. The same goes for the sixth letter from your previous entry's second word.
When figuring out which letters to "forget", if a word is too short, overflow the count to the next word. If there's no next word, wrap around and start from the beginning of your entry. If you roll the same number on the dice OR if the dice end up pointing to the same letter, remove only one letter this time.
Keep going until you can no longer write. When this happens, your mutation is complete and the game is over.