What is the etymology of восемь and девять?
Восемь comes from PIE o̯ectou
Девять comes from PIE e̯neun
en, n̥, em, m̥ -> ę (and later -> Russian я) change is common for Slavic, so regularly it should produce (accounting for the ti-suffix) невять rather than девять, but the influence of the word for ten, десять changed the initial consonant to d-.
Similarly the word for eight came under influence from the word for seven.
Both words come from Indo-European roots.
It is quite easy to see it if you use Baltic/Germanic languages as proxy.
For example, Latvian/Lithuanian astoņi/aštuonì have roots in Proto-baltic aštō-, which apparently is very similar to Germanic -acht and comes down to the Indo-European oḱtṓ-. Considering things above the Old Church Slavonic osmĭ for "eight" fits beautifully into set. "os" just got "в" added at the front which is typical for Belarussian language and Ukrainian language (not sure if for the whole or just its dialects).