Where does “смерте́льный” come from?

смерте́льный (Russian) comes from Polish śmiertelny, from Old Czech smrtelný, from Old Czech smrtedlný.

Ancestry of “смерте́льный”, step by step

Step	Language	Word	Meaning
1	Polish	śmiertelny	lethal, deadly; mortal
2	Old Czech	smrtelný	—
3	Old Czech	smrtedlný	—

смерте́льный — Etymology, Origin & Word History | Etymology Explorer