Puzzle #7

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Across
  1. 1. Repetition of initial sounds in a phrase
  2. 3. Extremely luminous active galactic nucleus
  3. 7. Sachh is _____
  4. 8. Small remaining trace of something
  5. 10. Mythical creature with a riddle
  6. 14. Notes of a chord played in sequence
  7. 16. Famous actor known for his role in Sherlock Holmes
  8. 17. Underground burial chamber
  9. 20. Puzzle where the first letters spell a word or phrase
  10. 23. Lowest layer of Earth's atmosphere
  11. 25. Hired soldier working for pay
  12. 27. Freshwater fish
  13. 28. Device creating shifting symmetrical patterns
  14. 31. Optical illusion in the desert
  15. 34. Mythical half-human, half-horse creature
  16. 35. Food of the gods in mythology
  17. 37. Intricate network of passages
  18. 38. Pen name used by an author
Down
  1. 2. Book of synonyms
  2. 4. Chain of islands
  3. 5. Well known author Agatha (One of your favourites I think)
  4. 6. Tall, four-sided stone monument
  5. 9. Difficult problem or puzzle
  6. 11. Musical instrument with wooden bars
  7. 12. Whirling mass of air or water
  8. 13. One skilled in creating or solving crosswords
  9. 15. Sound frequency or sports field
  10. 18. Ancient counting device
  11. 19. Day when day and night are of equal length
  12. 21. Clever plan or scheme
  13. 22. Mythical creature made of various animals
  14. 24. Colony of nesting birds or seals
  15. 26. Ancient Egyptian writing system
  16. 29. Relating to a transitional state
  17. 30. Spotted wildcat of Central and South America
  18. 32. Word formed by rearranging letters of another
  19. 33. Sea named for its floating seaweed
  20. 36. Humorous five-line poem