Overwhelming amounts of radiation will destroy your body on the cellular level. The thing that kills you first is central nervous system failure. You will not be able to breathe eventually and die.
It will feel like getting a sunburn I’m pretty sure, at least that is how cancer patients undergoing radiotherapy treatments describe it. They do not get lethal doses of course but pretty high ones and it irritates the skin.
A lengthier explanation: we measure radiation in units of J/kg deposited. This is called a Gray, or Gy. Diagnostic medical procedures like chest x-ray, CT and mammograms are in the uGy-mGy range. Cancer treatment is in the many cGy range per day, 5 days per week. After all, we’re trying to kill the cancer.
Now big doses: 1-2 Gy will probably not harm you very badly aside from raising your risk of developing cancer ten years from now all of a sudden.
4-6 Gy will start to destroy your digestive system but with medical care you will make it out alive. Your blood is affected the most but like I said if you’re kept in a sterile environment and don’t get sick you can survive.
8-12 Gy is somewhere in the lethal range and you’re gonna have a bad time. You will vomit, have diarrhea and dry up. Your body can’t absorb water and nutrients when your small intestines aren’t working. This is the worst way to go.
15+ Gy and you’re a gonner from acute radiation sickness.
There was a Russian double agent or something from a few years ago that ingested coffee with radioactive polonium and he got the 8-12 Gy range dose in his belly and died a slow death.
I don’t remember his name but he basically called out Russia’s involvement on his deathbed and died of starvation.