It's taking everyone's favorite alicorn princess a bit more effort than expected to force a rather large stallion all the way down her tight throat but her greedy little stomach didn't let some pesky problem like Cadance needing an extra swallow or two taking some time stop it from getting to work! It's getting bleaker and bleaker for Sombra in there by the second as he gets a first-hoof "taste" of just how brutally efficient alicorn digestion truly is…
As for how he fit in there? Tight as it looks, it turns out alicorns are well adapted for consuming things larger than themselves. It comes in real handy for fueling their bodies' incredible energy demands. That final little throat bulge as Cadance finishes her final gulp is the last remnant of the once-formidable King Sombra the world will ever see, even if he technically still has a minute or three to live inside that somehow-still-svelte belly.