Location: quite a ways west of Cloud Ruler Temple (link goes to the Daedric Shrines Map at UESP)
Minimum Level: none, but figure on 20+
Offering: complete all other Daedric Quests
Reward: Oghma Infinium
Objective: soul trap one of each of the ten races
Aside from the fact that most gamers love the Oghma Infinium, this quest is here as a protection for gamers who don't mess with the Main Quest immediately. So they may have already completed all of the other Daedric artifact quests and disposed of the artifacts somehow. Then, in the "Blood of the Daedra" quest, Martin asks for a Daedric artifact and there aren't any to he had. Wrong-o. There's this one because you can't start this quest until the "Blood of the Daedra" quest is active. It's a sorry use for such a nifty artifact, but them's the breaks. Additionally, you must have completed all of the other Daedric quests (you can still do this one if you kept Umbra in Clavicus Vile's quest). Since you have to be level 20 to get Hircine's quest, that's probably the earliest practical timeframe for starting this one.
When all of the conditions are met, you'll be approached by Casta Flavus with Mora's quest: soul trap one of each of the races. You'll get Hermaeus Mora's Soul Gem and a 120-second Soul Trap spell to go with it. You can also go directly to the shrine - just follow the road behind Cloud Ruler.
So where to come up with all of these souls? There are a few possibilities that won't get you in trouble with the law:
Be very careful with this quest so that you never have more than one soul trap spell running at a time (this includes casting it twice on the same NPC before the spell duration expires). There's a serious hiccup that can cause the quest to fail. One at a time, please.
Your reward at completion is the Oghma Infinium. If you waited until too late to start the "Blood of the Daedra" quest and don't have another artifact that you're willing or able to sacrifice, this will have to be it. When you use the Oghma, you get to choose one of three paths: Steel, Shadows or Spirits.
Once you use the Oghma, it disappears. So if you have no other artifact to give Martin and use the Oghma, you're, like, totally hozed for the Main Quest and will not be able to complete it without cheating.