You may reveal the first card you draw each turn as you draw it. Whenever you reveal an instant or sorcery card this way, copy that card and you may cast the copy. That copy costs 2 less to cast.
When God-Eternal Kefnet dies or is put into exile from the battlefield, you may put it into it's owner's library third from the top.
Quote from Yanhko »It doesn’t ignore timing restrictions. The effect isn’t a until end of turn effect. You can cast it, but only if that’s a legal time to cast that card.
War of the Spark Release Notes
Compiled by Eli Shiffrin, with contributions from Laurie Cheers, Carsten Haese, Nathan Long, and Thijs van Ommen
Document last modified April 22, 2019
If you somehow control more than one God-Eternal Kefnet (perhaps because one is a Spark Double), you may reveal a card you draw for any number of their abilities. One at a time, each will copy the card if it’s an instant or sorcery, and you may cast each of them. Each copy resolves before you cast the next, and each copy’s cost is reduced by only 2.