Translation:The young man wants to descend from the altar.
"Climb down from" needs to be accepted, as well as "descend;" as does "youth" for "young man."
It's also a problem (in my opinion, at least) that "descendere de + abl" has to be rendered (currently) as "to descend FROM," since "to descend the altar" is equiv. in English to "get down from the altar." At this stage, Duolingo clearly needs a word-for-word rendering, that includes the preposition de.
Yes: just as Duolingo has "rise" as well as "climb up" for ascendere (I don't know if they accept "ascend"), it would be great to have "climb down" and "get down" as well as "descend," for descendere.
"Introibo ad altare Dei, ad Deum qui laetificat juventutem meam"? Is there anywhere you can hear that these days? The last Catholic Mass I came across was during a visit to Paris, years ago. It was just beginning as we entered Notre Dame. After recent events I doubt if there is much chance of a Tridentine Mass there anytime soon.
Nah, the Romans didn't practice human sacrifice except that one time when they buried a Vestal Virgin, which was probably a short revival. The gladiator games admittedly was originally human sacrifice, a funerary rite that became a public spectacle, but in the latter form it was very rarely fought to the death. Gladiatores were expensive, so you save them as much as possible, and likely even give them medical care.
There are actually examples of Roman writers being shocked by human sacrifice. Despite his own brutality, Julius Caesar wrote of the Druidic Wicker Man with apparent revulsion. Carthaginian practices too were condemned.
However, doesn't the strangle of the captive during the Roman Triumph kinda like human sacrifice? https://www.youtube.com/watch?v=F-VjCLR5L-c&feature=youtu.be&t=878
In brief: ab = "away from" ; dē = "down from"; ex = "out of." All 3 require ablative-case objects.
Note also that a very common use of dē is the meaning "concerning, about," especially as used in book-titles: Dē Rērum Nātūrā , "On the Nature of Things" (by Lucretius), Dē Rē Pūblicā , "About the Republic" (by Cicero), Dē Bellō Gallicō , "Concerning the War in Gaul" (by Caesar), etc.
When Latin lost its genitive case (the possessive "of" function), dē seems to have taken this over; at least, that seems to describe how the descendants of dē are used in Romance languages.