Is that chunk solid or squishy?
It's good to cut those open to see what's inside.
Sometimes eggs can have a bit of blood in them, or meat spots, or blood spots.
Caused from a piece of tissue or a blood vessel breaking as the ova is released.
We don't see those in grocery eggs because they are inspected and sorted out.
What you have there could be a big clot of blood or a chuck of pus from an infection called a 'lash egg'.
Could never happen again, or repeat indicating a health issue.
It'll sure make you break eggs one at a time into a dish,
rather than into your pan or recipe for a good long while.