Divide the artifacts into two tiers, endgame quality and non-endgame quality. There is some milestone in the game, possibly receiving or completing the Quest, before which you are much more likely to get non-endgame-quality artifacts and after which you are much more likely to get endgame-quality artifacts.

Alternatively, rather than making it a fixed milestone, allow the player to expend some sort of midgame-level resource to “bless” or “consecrate” a coaligned altar which guarantees that the next gift you get from it will come from the endgame-quality tier. There should be some limit on the amount of times this can happen, though. The resource is unspecified, but could be some sort of non-randomly-generated “ancient holy water” that appears in a few guaranteed places.