How can I get the Snowball artifact?

The snowball artifact (and all other Christmas-associated items) were part of a time-limited Christmas event. As mentioned in a December update on Steam:

Those of you have unlocked Hard Mode will be able to find new Snowball artifacts. Use Snowballs in a Beyond recipe to access the special Christmas stage.

They clarify in a later update that the drops will be available again, but I cannot tell if they include the snowballs and associated stage or only the holiday rewards in their idea of what's available:

We wanted to let you know that DemonCrawl's Christmas Event will be coming to a close soon, on Tuesday, December 31st at around 4 PM EST... There are only a couple days left to unlock your special holiday items! If you miss this opportunity, worry not - you will be able to find the same items again next year in the form of Beyond Mode drops.