12.1.9Flying, Trample During your upkeep each player puts a counter on a creature they control. Creatures with counters on them are Mutants in addition to their other creature types.

During your upkeep each player sacrifices a mutant creature. Demon Mutant can't be sacrificed this way.

During your upkeep if you can not sacrifice a mutant creature, tap Demon Mutant and it deals 6 damage to you.

The laboratory had to be closed down. Not because of any danger but because the lab ran out of test subjects.

Why not just "Creatures are mutants in addition to their other types." and "During your upkeep, each player sacrifices a creature other than ~. Unless you do, tap ~ and it deals 6 damage to you."

I'm hoping to integrate more "mutant creature matters" into the set which would interact with cards like this which would be turning creatures into mutants. I think it's intereting a creature turns into a mutant but then doesn't have to be the one sacrificed so it could actually end up helping a player out if they need one of their creatuers to be considered a mutant for whatever reason.

Can still be tweaked later on, maybe it really is just too much text. Can also always be a good opportunity to divide up a card into two seperate cards.