You can see a list of default dueling spirits under dueling/artifacts/default_artifacts.lua. If you want to create your own custom dueling spirits, it is advisable to do this in a new file under the /artifacts/ directory.

wOS.ALCS.Dueling.Artifact:RegisterArtifact({
    Name = "Sigma's Superiority", -- Name of the artifact
    Description = "Bask in the glory of the almighty Sigma", -- Description of the artifact
    RarityName = "Legendary", -- Rarity Name
    RarityColor = Color( 255, 125, 0 ), -- Rarity Color
    Model = "models/artifacts/sigmassuperiority", -- Artifact ModelID
    DropRequirement = 5, -- Ascension level required to drop this artifact
    OnSpawn = function( ply ) --
        ply:AddArtifactSkill( "Combat", 1, 1 ) -- Give a skill tree perk
        ply:SetMaxHealth( ply:GetMaxHealth() * 1.2 ) -- Increase their max health by 20%
        ply:SetHealth( ply:GetMaxHealth() ) -- Restore them to full health
    end,
})