Serious.sam.siberian.mayhem.merry.christmas.v1.... Instant

: Introduced alongside the holiday content, this mode allows you to restart the campaign with all your weapons, skills, and gadgets while introducing tougher enemy spawns and weapon upgrades. Related Gameplay Improvements (v1.0.4)

If you were specifically looking for features from the update (which arrived earlier in August 2022), it added: Guide :: Christmas event: XPMR "Freezer" Raygun weapon skin Serious.Sam.Siberian.Mayhem.Merry.Christmas.v1....

A "solid feature" of the (Patch 1.06) for Serious Sam: Siberian Mayhem is the permanent unlock for the Christmas Event via a new New Game+ perk . Key Christmas Features : Introduced alongside the holiday content, this mode

: A unique cosmetic for the XPMR "Burner" Raygun, unlockable by finding a Christmas Tree and completing a special portal challenge on each of the game's five levels. : Introduced alongside the holiday content