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A long sword resting in a sheath with elaborate carvings.
Literal
Elaborate carving [genitive-の] done-[resultative-てある] sheath [in-に] resting long-sword.
A descriptive noun phrase with no main verb — a caption or item description. 凝った (こった, 'elaborate / intricate,' from 凝る 'to be particular about / to be elaborate') modifies 彫刻 ('carving'). ~のしてある ('with X done / having X applied') is the resultative ~てある form, indicating the carvings were intentionally placed. 鞘 (さや, 'sheath/scabbard') and 長剣 (ちょうけん, 'long sword') are literary/historical vocabulary. 収まる (おさまる, 'to fit into / to be housed in') completes the image.