Are death eaters the only ones who know how to send up the dark mark? I ask because when it was sent up during the Quidditch World Cup, it was implied that only the death eaters knew how to do it.

However when Dumbledore went to recruit Slughorn and said that he didn’t send up the dark mark, Slughorn said that he knew he forgot something. Slughorn could almost certainly fake it. He knew enough about the dark arts to offer useful insight on horcruxes to a young Voldemort, This seems to imply that he knew how to conjure it. I always figured that Slug horn would have enough skills to set a fake dark mark. He won't use the exact spell that we heard Jr. use. But, instead, he'll use some conjuring spell to do an imitation, which would presumably have different properties