Maintenance for the week of November 17:
• [COMPLETE] Xbox: NA and EU megaservers for maintenance – November 19, 4:00AM EST (9:00 UTC) - 12:00PM EST (17:00 UTC)
Different strikes will give you a different boss, and only certain ones award the fragment.
If the final boss is a Grievous Twilight and some other Daedra, you won't get a fragment. If the boss is a Shackled Titan with a cultist (I've seen a Nord woman with an unavailable hairstyle, a Dunmer man, and a Khajiit woman), then you will get one.
It seems that you can tell which you're getting if you see it spawn. When spawning, there are few civilians and guild members there who get trapped. Every one I've seen, if there are only two people (a guild member and a civilian), I'll get the Twilight. But if there are three (one guild member and two civilians), it will be one that gives a cultist and therefore a fragment.