Definition of crucible
a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures:
the crucible tipped and the mould filled with liquid metal
a situation of severe trial, or in which different elements interact, leading to the creation of something new:
their relationship was forged in the crucible of war
So based of what is written above I suggest:
(суровое) испытание вечностью