H2H-Xmas is a lot of things. It is the sequel to Xmas Doom, a small 1994 Christmas-themed minisode that detailed Doomguy as the savior of Christmas. Indeed, some of the Xmas Doom personnel contributed to H2H-Xmas, most notably Joe Wilcox of Simply Silly Software, who went on to do the most reviled Duke Nukem 3D expansion, the similarly-themed Nuclear Winter. It is also one of Doom's first community megaWADs, though it was built on the foundation of the H2HMud competition, which challenged demo recorders to test their mettle in maps built by David Swift (and a few others), led by the Head to Head (H2H) Gaming Network. H2H never really caught on, though, and the ultra-hard level pack that resulted would have languished in obscurity... were it not for H2H-Xmas.
