Given the structure (five “words,” the last one llandrwyd looking like a Welsh place name, possibly Llandrwyd ), a likely solution is (A↔Z, B↔Y, etc.), which is common for such puzzles.

Alternative: Could be a (each letter moved one key on QWERTY). Test: t → y (no).

Let’s test Atbash on the first word: thmyl → t(20) ↔ g(7), h(8) ↔ s(19), m(13) ↔ n(14), y(25) ↔ b(2), l(12) ↔ o(15) → gsnbo — not obviously English.

Given the subreddit / puzzle context: thmyl might be “myth” + l? Actually, thmyl could be “thymol” (chemical) missing O.