The Mountain is suffering from the effects of Death Head Manticore poison.
Geege Schuman stashed this in Game of Thrones
“The flesh mortifies and the wounds ooze pus. Even maggots will not touch such foulness. His convulsions are so violent that I have had to gag him to prevent him from biting off his own tongue. I have cut away as much tissue as I dare, and treated the rot with boiling wine and bread mold, to no avail. The veins in his arms are turning black. When I leeched him, all the leeches died.”- Pycelle to the small council
If he dies, what happens to Tyrion? Because both would have died, so no clear winner.
Couple of things:
Doesn't matter; Tyrion's champion was defeated in combat.
Tyrion killed his father, the Hand of the King, during his escape. He's a murderer in truth now.
The Mountain won't die.