I changed my Entity registry code to:
int ArtifactID = EntityRegistry.findGlobalUniqueEntityId();
EntityRegistry.registerGlobalEntityID(Artifact.class, "AncientArtifact", ArtifactID);
EntityRegistry.registerModEntity(Artifact.class, "AncientArtifact", ArtifactID, this.instance, 0, 20, false);
The Entity stays in world now.
I'm re-using the RenderEnderCrystal class to render my entity, but it isn't rendering anything.
RenderManager renderer = Minecraft.getMinecraft().getRenderManager();
RenderingRegistry.registerEntityRenderingHandler(Artifact.class, new RenderEnderCrystal(renderer));