'lacus Curtius' dicitur a nomine equitis cuiusdam qui se...

[pmda]

In LLPSI Pars II: Roma Aeterna Orberg scribit:

Postremus siccatus est locus in medio foro qui 'lacus Curtius' dicitur a nomine equitis cuiusdam qui se armatum cum equo suo in voraginem ibi factam praecipitavisse narratur.

...post 'cuiusdam'.

..qui se armatum (who armed) cum equo suo (with his horse into a hole that had appeared / happened -'factam' <--> 'voraginem') there, was said to have fallen.

Cur 'se'? Eques in voraginem equitavisse voluerat?

Necessarius est 'se'?

[adrianus]

Est.

"qui se armatum praecipitavisse narratur" = "who is said to have fallen armed/in armour"

De "qui se armatum...praecipitavisse narratur" sine "se" pronomine, separatim cum verbis ut velle et similibus, roges quis fuerit qui equitaverit. Et cum "velle" subjectum accusativum et reflexum poni potest. Subjectum istîc requiritur.

Verbs such as "velle" and similar ones often use a complementary infinitive on its own but otherwise you need a subject with the infinitive in reported speech to make clear what the subject is of the clause. And even the "velle" verbs can take a reflexive subject accusative in indirect speech. (A&G §580.)

[timeodanaos]

He didn't fall into the pond, he threw himself into it!

I don't know the grammar you refer to, Adrianus, but 'se' is the object of praecipitavisse, 'qui' being the subject of both 'narratur' and in the nominativus cum infinitivo.

[adrianus]

He didn't fall into the pond, he threw himself into it!

Ah, you are right, of course, timeodanaos. I was wrong.
Rectè dicis, certum est, timeodanaos. Erravi.