Back to Harris, as for the Doom mods he made (also known as «WADs» in the Doom modding scene), one of his most well-known ones would be U.A.C. Labs, a two-level WAD that Eric Harris published in 1996 and is probably the most famous of the six levels that are not lost. What I think helps this level stand out is that it's simply the only one that we have that can be played single-player, while the others are all DM maps. Also, and this might get me burnt to the stake for saying this, but the level itself really isn't that bad, like, it's not super amazing, but if I played this without knowing who made it, I would have thought that it was one of the better Doom WADs from the era and shows me genuine creativity from the author who clearly knew some Doom mechanics well, such as in-fighting and creative use of teleporters (even if they don't always work correctly).
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Back to Harris, as for the Doom mods he made (also known as «WADs» in the Doom modding scene), one of his most well-known ones would be U.A.C. Labs, a two-level WAD that Eric Harris published in 1996 and is probably the most famous of the seven levels that are not lost. What I think helps this level stand out is that it's simply the only one that we have that can be played single-player, while the others are all DM maps. Also, and this might get me burnt to the stake for saying this, but the level itself really isn't that bad, like, it's not super amazing, but if I played this without knowing who made it, I would have thought that it was one of the better Doom WADs from the era and shows me genuine creativity from the author who clearly knew some Doom mechanics well, such as in-fighting and creative use of teleporters (even if they don't always work correctly).