Welcome, @caragiver1.
It can be challenging (and frightening) to find easy to understand information about rarer types of cancer. I found this website to get you started.
- Sarcomatoid renal (kidney) cancer https://www.cancerresearchuk.org/about-cancer/kidney-cancer/stages-types-grades/types-grades/sarcomatoid-renal
"Sarcomatoid renal cancer is a rare type of kidney cancer.
Most kidney cancers start in cells called clear cells, but they can start in other cells too. Almost any type of renal cell cancer can become sarcomatoid. This means that the cells of the cancer look like the cells of a sarcoma (cancer of the connective tissues, such as muscles, nerves, fat, blood vessels and fibrous tissue)."

Keytruda (pembrolizumab) is often discussed in the lung cancer group. You might find these discussions useful:
- Started immunotherapy maintenance: What side effects can I expect? https://connect.mayoclinic.org/discussion/keytrudaalimta-maintenance-side-effect/
- Immunotherapy: Keytruda (pembrolizumab) https://connect.mayoclinic.org/discussion/immunotherapy-and-chemo/

@formydad might also have some insights to share.

Caragiver, when did you husband start treatment? Will this be a maintenance treatment, in others words, treatment continues rather than a set number of treatments. And, most importantly, how are you doing?