When I'm bored, I like to conceive an evil conlang with 8 declensions, 7 types of mutations, a pitch accent, and a shitload of prefixes

So:
"Kæn" (a region) → "Kænyù" (regions)
"El-" (your) and "o-" (locative marker)
"El+kæn" → "Elǧænyù" (your regions)
"O+kæn" → "Oxænyù" (in regions)

"In your regions"?
"O+el+kænyù"
"O+elǧænyù"
"Oelǧænyù" doesn't work because of "OE" → "Oxelǧænyù"

It's evil, and you can do worse:
"Oxšanghelǧænyù"
"O-(x)-ša-(ngh)-el-(ǧ)æn-yù"
"In-(x)-those-(ngh)-your-region-s"
→ "In those regions of yours"
Yes, you have to link the prefixes with the group of letters the initial "k-" of "kæn" would become had they been prefixed on their own 😈